Add encoding to open() call

With addition of unspecified-encoding checker this call
needs an encoding.

tests/test_functional.py

@@ -59,7 +59,7 @@ def _check_output_text(self, _, expected_output, actual_output):
             if os.path.exists(self._test_file.expected_output):
                 os.remove(self._test_file.expected_output)
             return
-        with open(self._test_file.expected_output, "w") as f:
+        with open(self._test_file.expected_output, "w", encoding="utf-8") as f:
             writer = csv.writer(f, dialect="test")
             for line in actual_output:
                 writer.writerow(line.to_csv())