Make AsyncExitStack generic on __aexit__ (#13888)

Author: provinzkraut

stdlib/contextlib.pyi

@@ -179,7 +179,7 @@ class AsyncExitStack(_BaseExitStack[_ExitT_co], metaclass=abc.ABCMeta):
     async def __aenter__(self) -> Self: ...
     async def __aexit__(
         self, exc_type: type[BaseException] | None, exc_value: BaseException | None, traceback: TracebackType | None, /
-    ) -> bool: ...
+    ) -> _ExitT_co: ...
 
 if sys.version_info >= (3, 10):
     class nullcontext(AbstractContextManager[_T, None], AbstractAsyncContextManager[_T, None]):