Message 411143 - Python tracker

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> bpo-46417: Revert remove_subclass() change (GH-30750)

Ok, this change fixed buildbots.

I saw code in typeobject.c which uses a borrowed reference to tp_subclasses with a loop which can modify tp_subclasses. This code should be modified to hold a strong reference to tp_subclasses while accessing it.

The test_mock_add_spec() test of test_unittest modifies the subclasses of many types and so is indirectly a stress tests for code accessing tp_subclasses. That's why the regression was only seen in this specific test.