Add sockets attribute to asyncio.events.AbstractServer (#2364)

Author: ymyzk

stdlib/3/asyncio/events.pyi

@@ -34,6 +34,7 @@ class TimerHandle(Handle):
     def __hash__(self) -> int: ...
 
 class AbstractServer:
+    sockets: Optional[List[socket]]
     def close(self) -> None: ...
     @coroutine
     def wait_closed(self) -> Generator[Any, None, None]: ...