fix: update return value of Pool.__await__()

Author: bryanforbes

asyncpg-stubs/pool.pyi

@@ -600,7 +600,7 @@ class Pool(Generic[_Record]):
     async def close(self) -> None: ...
     def terminate(self) -> None: ...
     async def expire_connections(self) -> None: ...
-    def __await__(self) -> Generator[Any, None, Self | None]: ...
+    def __await__(self) -> Generator[Any, None, Self]: ...
     async def __aenter__(self) -> Self: ...
     async def __aexit__(self, *exc: object) -> None: ...