Implement PEP 702: Marking deprecations using the type system

[JelleZijlstra]

PEP-702 is still under consideration by the Steering Council. I'm opening a PR now with a draft implementation, so we can get it merged quickly if the SC approves the PEP in time for the 3.12 cutoff.

Linked PRs

• gh-104003: Implement PEP 702 #104004

[hugovk]

PEP approved and implementation merged! 🎉