Add UP rule: rewrite TypeVar defaults for Generator / AsyncGenerator

[hugovk]

There's a new check in pyupgrade to rewrite using PEP 696 TypeVar defaults:

• https://github.com/asottile/pyupgrade#pep-696-typevar-defaults
• https://github.com/asottile/pyupgrade/pull/948

from collections.abc import Generator, AsyncGenerator

-def f() -> Generator[int, None, None]:
+def f() -> Generator[int]:
     yield 1

from collections.abc import Generator, AsyncGenerator

-async def f() -> AsyncGenerator[int, None]:
+async def f() -> AsyncGenerator[int]:
     yield 1

See the https://github.com/asottile/pyupgrade/pull/948/files for some variations.

[iamlucasvieira]

May I work on this one?

[iamlucasvieira]

I think we already have this rule implemented: UP043