docs: def _subscribe(self):
          """
          Start the subscription to the channel list.
          If self._keep_alive_function isn't None start timer thread to
          run self._keep_alive_function every self._keep_alive amount of seconds.
          """
          _LOGGER.info("PubNub subscribing")
          self._pubnub.subscribe().channels(CHANNELS).execute()
          if self._keep_alive_function is not None:
              threading.Timer(self._keep_alive, self._run_keep_alive).start()
          self._subscribed = True