gh-140873: Add support of non-descriptor callables in functools.singledispatchmethod()

[serhiy-storchaka]

• Issue: Allow non-descriptor callables in functools.singledispatchmethod #140873

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📚 Documentation preview 📚: https://cpython-previews--140884.org.readthedocs.build/

[emmatyping]

Could you please also add a regression test where singledispatchmethod decorates a descriptor method but a non-descriptor callable is registered like in the original issues? Just to make sure that never regresses.

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[emmatyping]

Thanks! This looks good.