expand: Give bytes to ast.parse to let it discover encoding cookie.

changelog.d/3613.misc.rst

@@ -0,0 +1 @@
+Fixed encoding errors in ``expand.StaticModule`` when system default encoding doesn't match expectations for source files.

setuptools/config/expand.py

@@ -21,6 +21,7 @@
 import importlib
 import io
 import os
+import pathlib
 import sys
 import warnings
 from glob import iglob
@@ -62,9 +63,7 @@ class StaticModule:
     """Proxy to a module object that avoids executing arbitrary code."""
 
     def __init__(self, name: str, spec: ModuleSpec):
-        with open(spec.origin) as strm:  # type: ignore
-            src = strm.read()
-        module = ast.parse(src)
+        module = ast.parse(pathlib.Path(spec.origin).read_bytes())
         vars(self).update(locals())
         del self.self
 

setuptools/tests/config/test_expand.py

@@ -60,6 +60,20 @@ def test_read_files(tmp_path, monkeypatch):
 
 
 class TestReadAttr:
+    @pytest.mark.parametrize(
+        "example",
+        [
+            # No cookie means UTF-8:
+            b"__version__ = '\xc3\xa9'\nraise SystemExit(1)\n",
+            # If a cookie is present, honor it:
+            b"# -*- coding: utf-8 -*-\n__version__ = '\xc3\xa9'\nraise SystemExit(1)\n",
+            b"# -*- coding: latin1 -*-\n__version__ = '\xe9'\nraise SystemExit(1)\n",
+        ]
+    )
+    def test_read_attr_encoding_cookie(self, example, tmp_path):
+        (tmp_path / "mod.py").write_bytes(example)
+        assert expand.read_attr('mod.__version__', root_dir=tmp_path) == 'é'
+
     def test_read_attr(self, tmp_path, monkeypatch):
         files = {
             "pkg/__init__.py": "",