`typing`: `_PickleUsingNameMixin` is dead code

[AlexWaygood]

Bug report

Bug description:

The typing._PickleUsingNameMixin class is dead code, and should be removed. In Python 3.11, it was used as a mixin class for typing.TypeVar, typing.TypeVarTuple, and typing.ParamSpec. However, all three classes are now implemented in C on Python 3.12+, and no longer make use of this mixin.

Linked PRs

• gh-110180: Remove unused _PickleUsingNameMixin class from typing #110181