Fix mypy heisenfailure on Windows

Author: oschwald

tests/reader_test.py

@@ -522,7 +522,7 @@ def test_closed_metadata(self) -> None:
             self.assertIsNotNone(metadata, "pure Python implementation returns value")
 
     if os.name != "nt":
-
+        # type: ignore[attr-defined]
         def test_multiprocessing(self):
             self._check_concurrency(self.mp.Process)