builtins: add BaseExceptionGroup.__class_getitem__ (#7648)

Author: hauntsaninja

stdlib/builtins.pyi

@@ -1783,6 +1783,7 @@ if sys.version_info >= (3, 11):
         @overload
         def split(self: Self, __condition: Callable[[_BaseExceptionT_co], bool]) -> tuple[Self | None, Self | None]: ...
         def derive(self: Self, __excs: Sequence[_BaseExceptionT_co]) -> Self: ...
+        def __class_getitem__(cls, __item: Any) -> GenericAlias: ...
 
     class ExceptionGroup(BaseExceptionGroup[_ExceptionT_co], Exception):
         def __new__(cls: type[Self], __message: str, __exceptions: Sequence[_ExceptionT_co]) -> Self: ...