code: def _process_ping(self):
          
          yield self.send_command(PONG_PROTO)
          if self._flush_queue.empty():
              yield self._flush_pending()
docs: The server will be periodically sending a PING, and if the the client
          does not reply a PONG back a number of times, it will close the connection
          sending an `-ERR 'Stale Connection'` error.