Fix __class_getitem__ to be positional-only

[ankith26]

As noted by @aatle in #3398, __class_getitem__ is positional only, and stubtest on python 3.13 complains about this issue. This PR fixes that.

[gresm]

I've looked around and failed to find information about key parameter being required a positional-only:
PEP-560: original spec
Python documentation
Possible alternative that it's Generic stuff instead - also negative

Edit:
The cause is __class_getitem__ = classmethod(types.GenericAlias) line. What I said above is true, but when instantiated, GenericAliasexpects a positional-only argument. An alternative is to just subclass AbstractClass from a generic in sprite.py like stub pretends (I wonder why there's no warning about that...)

[aatle]

If you would like, you can rename generic to item to be consistent with typeshed.

LGTM;

[ankith26]

I updated the code to use item instead of generic as per the review.

I also want to keep code changes out of this stubs PR so I am not changing the parent classes in the implementation, though someone could take it up in the future if interested.

[Starbuck5]

I played around a bit with list's implementation, it being positional only seems to be standard.

>>> list.__class_getitem__(key=int)
Traceback (most recent call last):
  File "<python-input-9>", line 1, in <module>
    list.__class_getitem__(key=int)
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^
TypeError: list.__class_getitem__() takes no keyword arguments

Also ofc class getitem is meant to be used by type hints, and trying to use a keyword there is a syntax error.

>>> list[key=int]
  File "<python-input-5>", line 1
    list[key=int]
         ^^^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

[damusss]

LGTM, thanks!