workaround (pypa/setuptools#1042)

Author: be-hase

tox.ini

@@ -3,6 +3,7 @@ envlist = py27, py33, py34, py35, py36, py36-flake8-src, py36-flake8-other
 
 [testenv]
 deps =
+    six
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/requirements-test.txt
 commands = py.test -v tests/
@@ -15,12 +16,14 @@ commands = py.test -v --cov=linebot tests/
 [testenv:py36-flake8-src]
 basepython = python3.6
 deps =
+    six
     flake8
     flake8-docstrings
 commands = flake8 linebot/
 
 [testenv:py36-flake8-other]
 basepython = python3.6
 deps =
+    six
     flake8
 commands = flake8 tests/ examples/