gh-130095: Mark _py_abc tests as not thread-safe (gh-130131)

The `_py_abc` implementation is not currently thread-safe (even with the GIL).
Don't run these tests with `--parallel-threads=N` for now.

Author: colesbury

Lib/test/test_abc.py

@@ -684,10 +684,16 @@ class B(A, metaclass=abc_ABCMeta, name="test"):
 
     return TestLegacyAPI, TestABC, TestABCWithInitSubclass
 
-TestLegacyAPI_Py, TestABC_Py, TestABCWithInitSubclass_Py = test_factory(abc.ABCMeta,
-                                                                        abc.get_cache_token)
-TestLegacyAPI_C, TestABC_C, TestABCWithInitSubclass_C = test_factory(_py_abc.ABCMeta,
-                                                                     _py_abc.get_cache_token)
+TestLegacyAPI_Py, TestABC_Py, TestABCWithInitSubclass_Py = test_factory(_py_abc.ABCMeta,
+                                                                        _py_abc.get_cache_token)
+TestLegacyAPI_C, TestABC_C, TestABCWithInitSubclass_C = test_factory(abc.ABCMeta,
+                                                                     abc.get_cache_token)
+
+# gh-130095: The _py_abc tests are not thread-safe when run with
+# `--parallel-threads`
+TestLegacyAPI_Py.__unittest_thread_unsafe__ = True
+TestABC_Py.__unittest_thread_unsafe__ = True
+TestABCWithInitSubclass_Py.__unittest_thread_unsafe__ = True
 
 if __name__ == "__main__":
     unittest.main()