Implement PEP 696 (Type parameter defaults)

[JelleZijlstra]

Feature or enhancement

Implement PEP-696, which was just accepted.

Off the top of my head, we'll need the following:

• Grammar changes
• Changes in typing.py to type parameter application logic
• Documentation

I'll work on the first one but contributions on the other two are appreciated. The implementation in typing-extensions can serve as an inspiration.

cc @Gobot1234

Linked PRs

• gh-116126: Implement PEP 696 #116129