gh-89132: Fix subclassing of annotated types - #155360

Subclassing an annotated type only worked if the annotated type was a class:

>>> from typing import Annotated, List
>>> class X(Annotated[List[int], 'metadata']): pass
...
TypeError: _GenericAlias.__init__() takes 3 positional arguments but 4 were given
>>> class X(Annotated[list[int], 'metadata']): pass
...
TypeError: GenericAlias expected 2 arguments, got 3

__mro_entries__() returned the annotated type itself, but it can need a resolution too. It now delegates to __mro_entries__() of the annotated type, so subclassing Annotated[X, ...] is the same as subclassing X, including the error if X cannot be subclassed.

• Issue: Subclassing of annotated types does not always work #89132