Typing spec update: clarifications to user-defined generic classes

[rchen152]

Please consider the following update to the typing spec:

• PR: https://github.com/python/typing/pull/1879
• Discussion: https://discuss.python.org/t/clarifying-the-rules-for-subclassing-generic-classes/69698

TC Sign-off

• [x] Carl
• [x] Eric
• [x] Jelle
• [ ] Jukka
• [x] Rebecca

[rchen152]

@JelleZijlstra, @JukkaL do you have any additional thoughts on https://github.com/python/typing/pull/1879?

[JelleZijlstra]

Oops, I thought I'd already signed off.