use Self in async context managers (#5724)

stdlib/asyncio/events.pyi

@@ -1,6 +1,6 @@
 import ssl
 import sys
-from _typeshed import FileDescriptorLike
+from _typeshed import FileDescriptorLike, Self
 from abc import ABCMeta, abstractmethod
 from socket import AddressFamily, SocketKind, _Address, _RetAddress, socket
 from typing import IO, Any, Awaitable, Callable, Dict, Generator, List, Optional, Sequence, Tuple, TypeVar, Union, overload
@@ -57,7 +57,7 @@ class TimerHandle(Handle):
 class AbstractServer:
     def close(self) -> None: ...
     if sys.version_info >= (3, 7):
-        async def __aenter__(self: _T) -> _T: ...
+        async def __aenter__(self: Self) -> Self: ...
         async def __aexit__(self, *exc: Any) -> None: ...
         def get_loop(self) -> AbstractEventLoop: ...
         def is_serving(self) -> bool: ...