Typing spec update: clarifications to user-defined generic classes

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
* [x] Jukka
* [x] Rebecca