Add runtime support for pygame.sprite.AbstractGroup subscripts

[Matiiss]

Currently this code would fail at runtime

import pygame


class MySprite(pygame.sprite.Sprite):
    ...


my_group: pygame.sprite.Group[MySprite] = pygame.sprite.Group()

with

Traceback (most recent call last):
  File "C:\Users\Matiiss\PycharmProjects\Something\for_so.py", line 8, in <module>
    my_group: pygame.sprite.Group[MySprite] = pygame.sprite.Group()
              ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
TypeError: type 'Group' is not subscriptable

Currently a viable alternative is to use

my_group: "pygame.sprite.Group[MySprite]" = pygame.sprite.Group()

or

from __future__ import annotations

However, I feel as though that makes it more cumbersome for the end user.
Also this solves a cross compatibility issue with pg/pg

Besides this, the type stubs for pygame.sprite have to be massively revised, they are quite flawed it seems, especially when this Generic mechanism is taken into account, but I will handle that in a another PR.

[damusss]

LGTM, with this change one doesn't have to wrap the subscript inside a string for annotations.

I don't think importing typing inside the function is a problem because:

• The class is subscripted a little amount of times
• Sequential subscripts will use the cached module, so it's just like if you import it at the top of the file

Regarding the generic implementation, I think the sprite stubs do a good enough job with subclassing Generic and using the typevars, so I think the actual implementation can ignore all that and only implement class getitem for the runtime.

Thanks :)

[Starbuck5]

Now that we've dropped 3.8 is this possible to do without using undocumented features?

[Matiiss]

I was wondering whether some of this should be put into typing_sample_app.py to ensure that mypy is fine with this, but that file seems more targeted towards typing.py and there already are unit tests for this runtime generic support (though in testing redundancy is fine). Thoughts? Should I add some samples to typing_sample_app.py or is the current testing sufficient?