Fix Signal._live_receivers typing

`_live_receivers` returns a tuple of two lists of callables since [support for async](django/django@e83a885#diff-65f7ef82645dcdb6a7898f2126781336c222437f65d3666c9d65b89edf725a6bR456) has been added.

django-stubs/dispatch/dispatcher.pyi

@@ -26,7 +26,7 @@ class Signal:
     async def asend(self, sender: Any, **named: Any) -> list[tuple[Callable, str | None]]: ...
     def send_robust(self, sender: Any, **named: Any) -> list[tuple[Callable, Exception | Any]]: ...
     async def asend_robust(self, sender: Any, **named: Any) -> list[tuple[Callable, Exception | Any]]: ...
-    def _live_receivers(self, sender: Any) -> list[Callable]: ...
+    def _live_receivers(self, sender: Any) -> tuple[list[Callable], list[Callable]]: ...
 
 _F = TypeVar("_F", bound=Callable[..., Any])