Fix type of closing

Entering the `contextlib.closing` context manager returns its argument, not an instance of `contextlib.closing` itself.

Author: sean-anderson-seco

asyncstdlib/contextlib.pyi

@@ -60,7 +60,7 @@ def contextmanager(
 
 class closing(Generic[AClose]):
     def __init__(self, thing: AClose) -> None: ...
-    async def __aenter__(self: Self) -> Self: ...
+    async def __aenter__(self: Self) -> AClose: ...
     async def __aexit__(
         self,
         exc_type: type[BaseException] | None,