bpo-30830: logging.config.listen() calls server_close() (#3524)

The ConfigSocketReceiver.serve_until_stopped() method from
logging.config.listen() now calls server_close() (of
socketserver.ThreadingTCPServer) rather than closing manually the
socket.

While this change has no effect yet, it will help to prevent dangling
threads once ThreadingTCPServer.server_close() will join spawned
threads (bpo-31233).