Update transaction.pyi

[rayansostenes]

Make transaction.atomic compatible with AbstractContextManager protocol.

Currently the following error is reported by pyright:

from contextlib import AbstractContextManager
from django.db import transaction

x: AbstractContextManager[None, None] = transaction.atomic()

Type "Atomic" is not assignable to declared type "AbstractContextManager[None, None]"
  "Atomic" is incompatible with protocol "AbstractContextManager[None, None]"
    "__exit__" is an incompatible type
      Type "(exc_type: None, exc_value: None, traceback: None) -> None" is not assignable to type "(exc_type: type[BaseException] | None, exc_value: BaseException | None, traceback: TracebackType | None, /) -> _ExitT_co@AbstractContextManager"
        Parameter 1: type "type[BaseException] | None" is incompatible with type "None"
          Type "type[BaseException] | None" is not assignable to type "None"
        Parameter 2: type "BaseException | None" is incompatible with type "None"
          Type "BaseException | None" is not assignable to type "None"
        Parameter 3: type "TracebackType | None" is incompatible with type "None"
        

[sbdchd]

Thanks!