gh-139847: Add PyType_Lookup() function

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• Issue: [C API] Add PyType_Lookup() function #139847

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📚 Documentation preview 📚: https://cpython-previews--139848.org.readthedocs.build/

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Check labels / Unresolved review (pull_request): Failing after 6s

The CI fails with:

Error: Label error. Requires exactly 1 of: awaiting merge. Found: type-feature, awaiting core review, interpreter-core, topic-C-API

I added the awaiting merge label to repair this CI.

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I reverted changes to schedule _PyType_Lookup() and _PyType_LookupRef() removal, just to make this PR easier to review and more "atomic". I prefer to write a separated PR for the removal.