Implement PEP 661 Sentinel Values

[JelleZijlstra]

Feature or enhancement

Proposal:

PEP-661 has not been accepted (python/steering-council#258), but I hope to still get it in for 3.15, and in view of the short time window available I'd like to already submit a PR so those interested can review the implementation already while the SC deliberates.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-148829: Implement PEP 661 #148831

[hugovk]

And I'll be happy if any buildbot failures can be found before any last-minute merge on release day :)