Use PEP 661 (sentinels) in the standard library

Feature or enhancement

Proposal:

I just merged support for PEP-661, which adds the new sentinel() builtin. Let's use it in the standard library so we can simplify some code and stress-test the feature.

We'll have to be careful to preserve compatibility with features such as repr() and boolean values.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-149084
• gh-149086
• gh-149088
• gh-149176
• gh-149213
• gh-149536
• gh-149591
• gh-149592
• gh-149612
• gh-149613
• gh-149653
• gh-151017
• gh-155919
• gh-155927