Open file explicitly with UTF-8 so it works on Windows, too

tests/test_black.py

@@ -2062,7 +2062,7 @@ async def test_blackd_response_black_version_header(self) -> None:
         self.assertIsNotNone(response.headers.get(blackd.BLACK_VERSION_HEADER))
 
 
-with open(black.__file__, "r") as _bf:
+with open(black.__file__, "r", encoding="utf-8") as _bf:
     black_source_lines = _bf.readlines()