Support metaclasses in get_all_subclasses

[rupayon123]

The change

Call the built-in subclass descriptor with an explicit class argument so get_all_subclasses(type) and traversal from object do not raise TypeError on metaclasses. Existing traversal order and deduplication remain intact.

Both new regression tests fail before the fix; all 674 tests and module doctests pass on Python 3.11.15 afterward.

The backstory

This follows the utility-library investigation in #502. Subclass discovery through type or object is useful for class registries and introspection. This was found through source inspection and controlled regression tests.

AI assistance

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