types: add __getitem__ on GenericAlias (#9172)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

stdlib/types.pyi

@@ -585,6 +585,7 @@ if sys.version_info >= (3, 9):
         @property
         def __parameters__(self) -> tuple[Any, ...]: ...
         def __init__(self, origin: type, args: Any) -> None: ...
+        def __getitem__(self, __typeargs: Any) -> GenericAlias: ...
         if sys.version_info >= (3, 11):
             @property
             def __unpacked__(self) -> bool: ...