gh-149083: Make functools.Placeholder a sentinel

[JelleZijlstra]

This greatly simplifies the code. The only behavioral difference observable in the tests is that type(Placeholder)() no longer works, because Placeholder is now an instance of sentinel.

• Issue: Use PEP 661 (sentinels) in the standard library #149083

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📚 Documentation preview 📚: https://cpython-previews--149088.org.readthedocs.build/

[picnixz]

Is type(Placeholder)() acceptable? I would have actually expected that type(MySentinel)() returned the sentinel as for type(None)() but it's not the case since MySentinel is an instance of a sentinel!

[picnixz]

Actually, since we're launching beta, it may be ok to change this and see if it breaks something