The issue comes maybe from the complex code in Lib/test/libregrtest/refleak.py to handle the ABC cache:

    abcs = {}
    for abc in [getattr(collections.abc, a) for a in collections.abc.__all__]:
        if not isabstract(abc):
            continue
        for obj in abc.__subclasses__() + [abc]:
            abcs[obj] = _get_dump(obj)[0]

...

    # clear type cache
    sys._clear_type_cache()

    # Clear ABC registries, restoring previously saved ABC registries.
    abs_classes = [getattr(collections.abc, a) for a in collections.abc.__all__]
    abs_classes = filter(isabstract, abs_classes)
    for abc in abs_classes:
        for obj in abc.__subclasses__() + [abc]:
            for ref in abcs.get(obj, set()):
                if ref() is not None:
                    obj.register(ref())
            obj._abc_caches_clear()

    clear_caches()