diff --git a/.github/workflows/fluent.syntax.yml b/.github/workflows/fluent.syntax.yml
index 0ff16cc0..68c1dd4c 100644
--- a/.github/workflows/fluent.syntax.yml
+++ b/.github/workflows/fluent.syntax.yml
@@ -24,7 +24,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, pypy2, pypy3]
+        python-version: [3.6, 3.7, 3.8, 3.9, pypy3]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
@@ -35,7 +35,6 @@ jobs:
         run: |
           python -m pip install wheel
           python -m pip install --upgrade pip
-          python -m pip install six
       - name: Test
         working-directory: ./fluent.syntax
         run: |
@@ -47,7 +46,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.9
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
diff --git a/fluent.syntax/CHANGELOG.md b/fluent.syntax/CHANGELOG.md
index 05e0dc1f..0114b33a 100644
--- a/fluent.syntax/CHANGELOG.md
+++ b/fluent.syntax/CHANGELOG.md
@@ -1,5 +1,10 @@
 # Changelog
 
+## fluent.syntax 0.19 (TBD)
+
+  - Drop support for Python 2.7 and 3.5
+  - Add support for Python 3.6 through 3.9
+
 ## fluent.syntax 0.18.1 (September 15, 2020)
 
   - Fix serialization of multiline patterns starting with special characters. (#156)
diff --git a/fluent.syntax/fluent/syntax/ast.py b/fluent.syntax/fluent/syntax/ast.py
index 7ad5d611..acd5a1f8 100644
--- a/fluent.syntax/fluent/syntax/ast.py
+++ b/fluent.syntax/fluent/syntax/ast.py
@@ -1,9 +1,6 @@
-# coding=utf-8
-from __future__ import unicode_literals
 import re
 import sys
 import json
-import six
 
 
 def to_json(value, fn=None):
@@ -44,7 +41,7 @@ def scalars_equal(node1, node2, ignored_fields):
     return node1 == node2
 
 
-class BaseNode(object):
+class BaseNode:
     """Base class for all Fluent AST nodes.
 
     All productions described in the ASDL subclass BaseNode, including Span and
@@ -125,7 +122,7 @@ class SyntaxNode(BaseNode):
     """Base class for AST nodes which can have Spans."""
 
     def __init__(self, span=None, **kwargs):
-        super(SyntaxNode, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.span = span
 
     def add_span(self, start, end):
@@ -134,7 +131,7 @@ def add_span(self, start, end):
 
 class Resource(SyntaxNode):
     def __init__(self, body=None, **kwargs):
-        super(Resource, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.body = body or []
 
 
@@ -145,7 +142,7 @@ class Entry(SyntaxNode):
 class Message(Entry):
     def __init__(self, id, value=None, attributes=None,
                  comment=None, **kwargs):
-        super(Message, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.id = id
         self.value = value
         self.attributes = attributes or []
@@ -155,7 +152,7 @@ def __init__(self, id, value=None, attributes=None,
 class Term(Entry):
     def __init__(self, id, value, attributes=None,
                  comment=None, **kwargs):
-        super(Term, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.id = id
         self.value = value
         self.attributes = attributes or []
@@ -164,7 +161,7 @@ def __init__(self, id, value, attributes=None,
 
 class Pattern(SyntaxNode):
     def __init__(self, elements, **kwargs):
-        super(Pattern, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.elements = elements
 
 
@@ -174,13 +171,13 @@ class PatternElement(SyntaxNode):
 
 class TextElement(PatternElement):
     def __init__(self, value, **kwargs):
-        super(TextElement, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.value = value
 
 
 class Placeable(PatternElement):
     def __init__(self, expression, **kwargs):
-        super(Placeable, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.expression = expression
 
 
@@ -191,7 +188,7 @@ class Expression(SyntaxNode):
 class Literal(Expression):
     """An abstract base class for literals."""
     def __init__(self, value, **kwargs):
-        super(Literal, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.value = value
 
     def parse(self):
@@ -206,7 +203,7 @@ def from_escape_sequence(matchobj):
                 return c
             codepoint = int(codepoint4 or codepoint6, 16)
             if codepoint <= 0xD7FF or 0xE000 <= codepoint:
-                return six.unichr(codepoint)
+                return chr(codepoint)
             # Escape sequences reresenting surrogate code points are
             # well-formed but invalid in Fluent. Replace them with U+FFFD
             # REPLACEMENT CHARACTER.
@@ -235,14 +232,14 @@ def parse(self):
 
 class MessageReference(Expression):
     def __init__(self, id, attribute=None, **kwargs):
-        super(MessageReference, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.id = id
         self.attribute = attribute
 
 
 class TermReference(Expression):
     def __init__(self, id, attribute=None, arguments=None, **kwargs):
-        super(TermReference, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.id = id
         self.attribute = attribute
         self.arguments = arguments
@@ -250,41 +247,41 @@ def __init__(self, id, attribute=None, arguments=None, **kwargs):
 
 class VariableReference(Expression):
     def __init__(self, id, **kwargs):
-        super(VariableReference, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.id = id
 
 
 class FunctionReference(Expression):
     def __init__(self, id, arguments, **kwargs):
-        super(FunctionReference, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.id = id
         self.arguments = arguments
 
 
 class SelectExpression(Expression):
     def __init__(self, selector, variants, **kwargs):
-        super(SelectExpression, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.selector = selector
         self.variants = variants
 
 
 class CallArguments(SyntaxNode):
     def __init__(self, positional=None, named=None, **kwargs):
-        super(CallArguments, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.positional = [] if positional is None else positional
         self.named = [] if named is None else named
 
 
 class Attribute(SyntaxNode):
     def __init__(self, id, value, **kwargs):
-        super(Attribute, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.id = id
         self.value = value
 
 
 class Variant(SyntaxNode):
     def __init__(self, key, value, default=False, **kwargs):
-        super(Variant, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.key = key
         self.value = value
         self.default = default
@@ -292,41 +289,41 @@ def __init__(self, key, value, default=False, **kwargs):
 
 class NamedArgument(SyntaxNode):
     def __init__(self, name, value, **kwargs):
-        super(NamedArgument, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.name = name
         self.value = value
 
 
 class Identifier(SyntaxNode):
     def __init__(self, name, **kwargs):
-        super(Identifier, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.name = name
 
 
 class BaseComment(Entry):
     def __init__(self, content=None, **kwargs):
-        super(BaseComment, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.content = content
 
 
 class Comment(BaseComment):
     def __init__(self, content=None, **kwargs):
-        super(Comment, self).__init__(content, **kwargs)
+        super().__init__(content, **kwargs)
 
 
 class GroupComment(BaseComment):
     def __init__(self, content=None, **kwargs):
-        super(GroupComment, self).__init__(content, **kwargs)
+        super().__init__(content, **kwargs)
 
 
 class ResourceComment(BaseComment):
     def __init__(self, content=None, **kwargs):
-        super(ResourceComment, self).__init__(content, **kwargs)
+        super().__init__(content, **kwargs)
 
 
 class Junk(SyntaxNode):
     def __init__(self, content=None, annotations=None, **kwargs):
-        super(Junk, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.content = content
         self.annotations = annotations or []
 
@@ -336,14 +333,14 @@ def add_annotation(self, annot):
 
 class Span(BaseNode):
     def __init__(self, start, end, **kwargs):
-        super(Span, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.start = start
         self.end = end
 
 
 class Annotation(SyntaxNode):
     def __init__(self, code, arguments=None, message=None, **kwargs):
-        super(Annotation, self).__init__(**kwargs)
+        super().__init__(**kwargs)
         self.code = code
         self.arguments = arguments or []
         self.message = message
diff --git a/fluent.syntax/fluent/syntax/errors.py b/fluent.syntax/fluent/syntax/errors.py
index cd137871..36859d20 100644
--- a/fluent.syntax/fluent/syntax/errors.py
+++ b/fluent.syntax/fluent/syntax/errors.py
@@ -1,6 +1,3 @@
-from __future__ import unicode_literals
-
-
 class ParseError(Exception):
     def __init__(self, code, *args):
         self.code = code
diff --git a/fluent.syntax/fluent/syntax/parser.py b/fluent.syntax/fluent/syntax/parser.py
index 6731136c..fb296368 100644
--- a/fluent.syntax/fluent/syntax/parser.py
+++ b/fluent.syntax/fluent/syntax/parser.py
@@ -1,4 +1,3 @@
-from __future__ import unicode_literals
 import re
 from . import ast
 from .stream import EOF, EOL, FluentParserStream
@@ -25,7 +24,7 @@ def decorated(self, ps, *args, **kwargs):
     return decorated
 
 
-class FluentParser(object):
+class FluentParser:
     """This class is used to parse Fluent source content.
 
     ``with_spans`` enables source information in the form of
@@ -469,7 +468,7 @@ def get_escape_sequence(self, ps):
 
         if next == '\\' or next == '"':
             ps.next()
-            return '\\{}'.format(next)
+            return f'\\{next}'
 
         if next == 'u':
             return self.get_unicode_escape_sequence(ps, next, 4)
@@ -485,10 +484,10 @@ def get_unicode_escape_sequence(self, ps, u, digits):
         for _ in range(digits):
             ch = ps.take_hex_digit()
             if not ch:
-                raise ParseError('E0026', '\\{}{}{}'.format(u, sequence, ps.current_char))
+                raise ParseError('E0026', f'\\{u}{sequence}{ps.current_char}')
             sequence += ch
 
-        return '\\{}{}'.format(u, sequence)
+        return f'\\{u}{sequence}'
 
     @with_span
     def get_placeable(self, ps):
diff --git a/fluent.syntax/fluent/syntax/serializer.py b/fluent.syntax/fluent/syntax/serializer.py
index 7c1bb087..b6e16b1c 100644
--- a/fluent.syntax/fluent/syntax/serializer.py
+++ b/fluent.syntax/fluent/syntax/serializer.py
@@ -1,4 +1,3 @@
-from __future__ import unicode_literals
 from . import ast
 
 
@@ -32,7 +31,7 @@ def should_start_on_new_line(pattern):
     return False
 
 
-class FluentSerializer(object):
+class FluentSerializer:
     """FluentSerializer converts :class:`.ast.SyntaxNode` objects to unicode strings.
 
     `with_junk` controls if parse errors are written back or not.
@@ -83,11 +82,11 @@ def serialize_entry(self, entry, state=0):
 
 def serialize_comment(comment, prefix="#"):
     prefixed = "\n".join([
-        prefix if len(line) == 0 else "{} {}".format(prefix, line)
+        prefix if len(line) == 0 else f"{prefix} {line}"
         for line in comment.content.split("\n")
     ])
     # Add the trailing line break.
-    return '{}\n'.format(prefixed)
+    return f'{prefixed}\n'
 
 
 def serialize_junk(junk):
@@ -100,7 +99,7 @@ def serialize_message(message):
     if message.comment:
         parts.append(serialize_comment(message.comment))
 
-    parts.append("{} =".format(message.id.name))
+    parts.append(f"{message.id.name} =")
 
     if message.value:
         parts.append(serialize_pattern(message.value))
@@ -119,7 +118,7 @@ def serialize_term(term):
     if term.comment:
         parts.append(serialize_comment(term.comment))
 
-    parts.append("-{} =".format(term.id.name))
+    parts.append(f"-{term.id.name} =")
     parts.append(serialize_pattern(term.value))
 
     if term.attributes:
@@ -142,9 +141,9 @@ def serialize_pattern(pattern):
     content = indent_except_first_line(content)
 
     if should_start_on_new_line(pattern):
-        return '\n    {}'.format(content)
+        return f'\n    {content}'
 
-    return ' {}'.format(content)
+    return f' {content}'
 
 
 def serialize_element(element):
@@ -169,32 +168,32 @@ def serialize_placeable(placeable):
 
 def serialize_expression(expression):
     if isinstance(expression, ast.StringLiteral):
-        return '"{}"'.format(expression.value)
+        return f'"{expression.value}"'
     if isinstance(expression, ast.NumberLiteral):
         return expression.value
     if isinstance(expression, ast.VariableReference):
-        return "${}".format(expression.id.name)
+        return f"${expression.id.name}"
     if isinstance(expression, ast.TermReference):
-        out = "-{}".format(expression.id.name)
+        out = f"-{expression.id.name}"
         if expression.attribute is not None:
-            out += ".{}".format(expression.attribute.name)
+            out += f".{expression.attribute.name}"
         if expression.arguments is not None:
             out += serialize_call_arguments(expression.arguments)
         return out
     if isinstance(expression, ast.MessageReference):
         out = expression.id.name
         if expression.attribute is not None:
-            out += ".{}".format(expression.attribute.name)
+            out += f".{expression.attribute.name}"
         return out
     if isinstance(expression, ast.FunctionReference):
         args = serialize_call_arguments(expression.arguments)
-        return "{}{}".format(expression.id.name, args)
+        return f"{expression.id.name}{args}"
     if isinstance(expression, ast.SelectExpression):
         out = "{} ->".format(
             serialize_expression(expression.selector))
         for variant in expression.variants:
             out += serialize_variant(variant)
-        return "{}\n".format(out)
+        return f"{out}\n"
     if isinstance(expression, ast.Placeable):
         return serialize_placeable(expression)
     raise Exception('Unknown expression type: {}'.format(type(expression)))
@@ -214,7 +213,7 @@ def serialize_call_arguments(expr):
     named = ", ".join(
         serialize_named_argument(arg) for arg in expr.named)
     if len(expr.positional) > 0 and len(expr.named) > 0:
-        return '({}, {})'.format(positional, named)
+        return f'({positional}, {named})'
     return '({})'.format(positional or named)
 
 
diff --git a/fluent.syntax/fluent/syntax/stream.py b/fluent.syntax/fluent/syntax/stream.py
index 1f3852c8..6b38179e 100644
--- a/fluent.syntax/fluent/syntax/stream.py
+++ b/fluent.syntax/fluent/syntax/stream.py
@@ -1,8 +1,7 @@
-from __future__ import unicode_literals
 from .errors import ParseError
 
 
-class ParserStream(object):
+class ParserStream:
     def __init__(self, string):
         self.string = string
         self.index = 0
diff --git a/fluent.syntax/fluent/syntax/visitor.py b/fluent.syntax/fluent/syntax/visitor.py
index 491a7959..cae5baac 100644
--- a/fluent.syntax/fluent/syntax/visitor.py
+++ b/fluent.syntax/fluent/syntax/visitor.py
@@ -1,10 +1,7 @@
-# coding=utf-8
-from __future__ import unicode_literals, absolute_import
-
 from .ast import BaseNode
 
 
-class Visitor(object):
+class Visitor:
     '''Read-only visitor pattern.
 
     Subclass this to gather information from an AST.
@@ -22,7 +19,7 @@ def visit(self, node):
         if not isinstance(node, BaseNode):
             return
         nodename = type(node).__name__
-        visit = getattr(self, 'visit_{}'.format(nodename), self.generic_visit)
+        visit = getattr(self, f'visit_{nodename}', self.generic_visit)
         visit(node)
 
     def generic_visit(self, node):
@@ -43,7 +40,7 @@ def visit(self, node):
             return node
 
         nodename = type(node).__name__
-        visit = getattr(self, 'visit_{}'.format(nodename), self.generic_visit)
+        visit = getattr(self, f'visit_{nodename}', self.generic_visit)
         return visit(node)
 
     def generic_visit(self, node):
diff --git a/fluent.syntax/setup.cfg b/fluent.syntax/setup.cfg
index 6c7afb09..53b3d312 100644
--- a/fluent.syntax/setup.cfg
+++ b/fluent.syntax/setup.cfg
@@ -1,5 +1,5 @@
 [metadata]
-version=0.18.1
+version=0.19
 
 [bdist_wheel]
 universal=1
diff --git a/fluent.syntax/setup.py b/fluent.syntax/setup.py
index 411e912f..0b5f3aae 100755
--- a/fluent.syntax/setup.py
+++ b/fluent.syntax/setup.py
@@ -19,11 +19,12 @@
           'Development Status :: 3 - Alpha',
           'Intended Audience :: Developers',
           'License :: OSI Approved :: Apache Software License',
-          'Programming Language :: Python :: 2.7',
-          'Programming Language :: Python :: 3.5',
+          'Programming Language :: Python :: 3.6',
+          'Programming Language :: Python :: 3.7',
+          'Programming Language :: Python :: 3.8',
+          'Programming Language :: Python :: 3.9',
+          'Programming Language :: Python :: 3 :: Only',
       ],
       packages=['fluent', 'fluent.syntax'],
-      # These should also be duplicated in tox.ini and /.github/workflow/fluent.syntax.yml
-      tests_require=['six'],
       test_suite='tests.syntax'
       )
diff --git a/fluent.syntax/tests/syntax/__init__.py b/fluent.syntax/tests/syntax/__init__.py
index 5bb38076..56301df6 100644
--- a/fluent.syntax/tests/syntax/__init__.py
+++ b/fluent.syntax/tests/syntax/__init__.py
@@ -1,7 +1,5 @@
-from __future__ import unicode_literals
-
 import textwrap
 
 
 def dedent_ftl(text):
-    return textwrap.dedent("{}\n".format(text.rstrip()))
+    return textwrap.dedent(f"{text.rstrip()}\n")
diff --git a/fluent.syntax/tests/syntax/test_ast_json.py b/fluent.syntax/tests/syntax/test_ast_json.py
index 352b748a..bbfa524b 100644
--- a/fluent.syntax/tests/syntax/test_ast_json.py
+++ b/fluent.syntax/tests/syntax/test_ast_json.py
@@ -1,4 +1,3 @@
-from __future__ import unicode_literals
 import unittest
 
 from tests.syntax import dedent_ftl
diff --git a/fluent.syntax/tests/syntax/test_entry.py b/fluent.syntax/tests/syntax/test_entry.py
index 494785af..7e4e25f9 100644
--- a/fluent.syntax/tests/syntax/test_entry.py
+++ b/fluent.syntax/tests/syntax/test_entry.py
@@ -1,4 +1,3 @@
-from __future__ import unicode_literals
 import unittest
 
 from tests.syntax import dedent_ftl
diff --git a/fluent.syntax/tests/syntax/test_equals.py b/fluent.syntax/tests/syntax/test_equals.py
index c60a35a0..bd9ceb80 100644
--- a/fluent.syntax/tests/syntax/test_equals.py
+++ b/fluent.syntax/tests/syntax/test_equals.py
@@ -1,4 +1,3 @@
-from __future__ import unicode_literals
 import unittest
 
 from tests.syntax import dedent_ftl
diff --git a/fluent.syntax/tests/syntax/test_literal.py b/fluent.syntax/tests/syntax/test_literal.py
index dc4d709b..7aba643f 100644
--- a/fluent.syntax/tests/syntax/test_literal.py
+++ b/fluent.syntax/tests/syntax/test_literal.py
@@ -1,5 +1,3 @@
-# coding: utf-8
-from __future__ import unicode_literals
 import sys
 import unittest
 
diff --git a/fluent.syntax/tests/syntax/test_reference.py b/fluent.syntax/tests/syntax/test_reference.py
index c69b1073..53c771f6 100644
--- a/fluent.syntax/tests/syntax/test_reference.py
+++ b/fluent.syntax/tests/syntax/test_reference.py
@@ -1,6 +1,3 @@
-from __future__ import unicode_literals
-from six import with_metaclass
-
 import os
 import json
 import codecs
@@ -54,11 +51,11 @@ def test(self):
             if file_name in ('leading_dots', 'variant_lists'):
                 continue
 
-            test_name = 'test_{}'.format(file_name)
+            test_name = f'test_{file_name}'
             attrs[test_name] = gen_test(file_name)
 
         return type.__new__(mcs, name, bases, attrs)
 
 
-class TestReference(with_metaclass(TestReferenceMeta, unittest.TestCase)):
+class TestReference(unittest.TestCase, metaclass=TestReferenceMeta):
     maxDiff = None
diff --git a/fluent.syntax/tests/syntax/test_serializer.py b/fluent.syntax/tests/syntax/test_serializer.py
index 1f716bc7..ffdd93df 100644
--- a/fluent.syntax/tests/syntax/test_serializer.py
+++ b/fluent.syntax/tests/syntax/test_serializer.py
@@ -1,7 +1,5 @@
-from __future__ import unicode_literals
 import unittest
 
-import six
 from tests.syntax import dedent_ftl
 from fluent.syntax import FluentParser, FluentSerializer
 from fluent.syntax.serializer import serialize_expression, serialize_variant_key
@@ -18,10 +16,10 @@ def pretty_ftl(text):
     def test_invalid_resource(self):
         serializer = FluentSerializer()
 
-        with six.assertRaisesRegex(self, Exception, 'Unknown resource type'):
+        with self.assertRaisesRegex(Exception, 'Unknown resource type'):
             serializer.serialize(None)
 
-        with six.assertRaisesRegex(self, Exception, 'Unknown resource type'):
+        with self.assertRaisesRegex(Exception, 'Unknown resource type'):
             serializer.serialize(object())
 
     def test_simple_message(self):
@@ -456,10 +454,10 @@ def pretty_expr(text):
         return serialize_expression(expr)
 
     def test_invalid_expression(self):
-        with six.assertRaisesRegex(self, Exception, 'Unknown expression type'):
+        with self.assertRaisesRegex(Exception, 'Unknown expression type'):
             serialize_expression(None)
 
-        with six.assertRaisesRegex(self, Exception, 'Unknown expression type'):
+        with self.assertRaisesRegex(Exception, 'Unknown expression type'):
             serialize_expression(object())
 
     def test_string_expression(self):
@@ -517,10 +515,10 @@ def pretty_variant_key(text, index):
         return serialize_variant_key(variants[index].key)
 
     def test_invalid_expression(self):
-        with six.assertRaisesRegex(self, Exception, 'Unknown variant key type'):
+        with self.assertRaisesRegex(Exception, 'Unknown variant key type'):
             serialize_variant_key(None)
 
-        with six.assertRaisesRegex(self, Exception, 'Unknown variant key type'):
+        with self.assertRaisesRegex(Exception, 'Unknown variant key type'):
             serialize_variant_key(object())
 
     def test_identifiers(self):
diff --git a/fluent.syntax/tests/syntax/test_structure.py b/fluent.syntax/tests/syntax/test_structure.py
index 3f8789dc..e7650b88 100644
--- a/fluent.syntax/tests/syntax/test_structure.py
+++ b/fluent.syntax/tests/syntax/test_structure.py
@@ -1,6 +1,3 @@
-from __future__ import unicode_literals
-from six import with_metaclass
-
 import os
 import json
 import codecs
@@ -43,11 +40,11 @@ def test(self):
             if ext != '.ftl':
                 continue
 
-            test_name = 'test_{}'.format(file_name)
+            test_name = f'test_{file_name}'
             attrs[test_name] = gen_test(file_name)
 
         return type.__new__(mcs, name, bases, attrs)
 
 
-class TestStructure(with_metaclass(TestStructureMeta, unittest.TestCase)):
+class TestStructure(unittest.TestCase, metaclass=TestStructureMeta):
     maxDiff = None
diff --git a/fluent.syntax/tests/syntax/test_visitor.py b/fluent.syntax/tests/syntax/test_visitor.py
index acd34284..dc1dd228 100644
--- a/fluent.syntax/tests/syntax/test_visitor.py
+++ b/fluent.syntax/tests/syntax/test_visitor.py
@@ -1,5 +1,3 @@
-from __future__ import unicode_literals
-
 from collections import defaultdict
 import unittest
 
@@ -16,7 +14,7 @@ def __init__(self):
 
     def generic_visit(self, node):
         self.calls[type(node).__name__] += 1
-        super(MockVisitor, self).generic_visit(node)
+        super().generic_visit(node)
 
     def visit_Pattern(self, node):
         self.pattern_calls += 1
@@ -71,7 +69,7 @@ def test(self):
         )
 
 
-class WordCounter(object):
+class WordCounter:
     def __init__(self):
         self.word_count = 0
 
@@ -87,13 +85,13 @@ def __init__(self):
 
     def generic_visit(self, node):
         if not isinstance(node, (ast.Span, ast.Annotation)):
-            super(VisitorCounter, self).generic_visit(node)
+            super().generic_visit(node)
 
     def visit_TextElement(self, node):
         self.word_count += len(node.value.split())
 
 
-class ReplaceText(object):
+class ReplaceText:
     def __init__(self, before, after):
         self.before = before
         self.after = after
@@ -113,7 +111,7 @@ def __init__(self, before, after):
     def generic_visit(self, node):
         if isinstance(node, (ast.Span, ast.Annotation)):
             return node
-        return super(ReplaceTransformer, self).generic_visit(node)
+        return super().generic_visit(node)
 
     def visit_TextElement(self, node):
         """Perform find and replace on text values only"""
diff --git a/fluent.syntax/tox.ini b/fluent.syntax/tox.ini
index a81134c3..2ed33bdb 100644
--- a/fluent.syntax/tox.ini
+++ b/fluent.syntax/tox.ini
@@ -1,12 +1,10 @@
 # This config is for local testing.
 # It should be correspond to .github/workflows/fluent.syntax.yml
 [tox]
-envlist = py27, py35, py36, py37, pypy, pypy3
+envlist = py36, py37, py38, py39, pypy3
 skipsdist=True
 
 [testenv]
 setenv =
     PYTHONPATH = {toxinidir}
-deps =
-     six
 commands = ./runtests.py