Initial implementation of PEP 661 v2 (sentinels)

[JelleZijlstra]

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📚 Documentation preview 📚: https://cpython-previews--4.org.readthedocs.build/

[JelleZijlstra]

Open questions:

• What about __bool__? Lots of discussion on Discourse, I don't care too much but don't want this to derail the feature. This draft implementation says sentinels are always truthy.
• Should sentinels be weakrefable?
• Should the sentinel class be subclassable? My inclination is no.
• Should there be a C API like PySentinel_New? Probably.

[Viicos]

Open questions:

Gave thoughts on python/peps#4923 (comment).

[DanielNoord]

I only looked at the .rst file as my minimal knowledge of C won't help much here. There are a couple of double spaces in some of them, perhaps before merging quickly grep for " " in the diff to remove those :)

[JelleZijlstra]

Thanks for looking! I think the double spaces are mainly between sentences, that's very common in the docs. It doesn't affect rendering.