Add support for PEP 661 sentinels

[JelleZijlstra]

We should add a PEP 661-style Sentinel class; see https://discuss.python.org/t/pep-661-sentinel-values/9126/243

cc @Viicos @taleinat.

(For clarity: I won't work on this myself but I'll review a PR.)

[Viicos]

@taleinat I can look at it this week (or when it will be decided whether the module registry is kept) and port your draft implementation linked in the PEP there.