Skip free threading test for pure Python implementation

Author: rodrigobnogueira

tests/test_free_threading.py

@@ -20,6 +20,9 @@ def test_race_condition_iterator_vs_mutation(
     a standard Python `RuntimeError` ('MultiDict is changed during iteration'), preventing
     crashes.
     """
+    if cls.__module__ == "multidict._multidict_py":
+        pytest.skip("Test is only applicable to the C extension")
+
     md: Union[CIMultiDict[str], MultiDict[str]] = cls()
     for i in range(8):
         md[f"init-{i}"] = f"v{i}"