gh-130870: Preserve GenericAlias subclasses in typing.get_type_hints()

[Viicos]

This is an alternative approach to #130897, that avoids recreating GenericAlias instances two times (no noticeable performance improvement but cleans up the implementation).

It also fixes the GenericAlias reconstruction in _strip_annotations(), and as such I moved the logic into a separate function.

This approach has the benefit of not losing the __unpacked__ property of the GenericAlias if no forward annotation is present in the type hint. That is:

def func(*args: *tuple[int]): ...

get_type_hints(func)
# On main:
# {'args': typing.Unpack[tuple[int]]}
# On this branch:
# {'args': *tuple[int]} (that is, gth(func)['args'] is a the same `GenericAlias` instance, with `__unpacked__ = True`)

A couple tests are failing due to this, I'm not sure how we can test this? Maybe like:

def func2(*args: *tuple[int, str]): pass
self.assertEqual(gth(func2), func2.__annotations__)
self.assertTrue(gth(func2)['args'].__unpacked__)

• Issue: typing._eval_type is not preserving GenericAlias subclasses #130870