asyncio.Server.socket fixes (#5538)

* Move socket from AbstractServer to Server.
* Fix Server.socket type on Python 3.7+.
* Use Iterable instead of list for socket argument.

Closes: #5535

stdlib/asyncio/events.pyi

@@ -53,7 +53,6 @@ class TimerHandle(Handle):
         def when(self) -> float: ...
 
 class AbstractServer:
-    sockets: Optional[List[socket]]
     def close(self) -> None: ...
     if sys.version_info >= (3, 7):
         async def __aenter__(self: _T) -> _T: ...