Ensure we have a SlackClient.

def _ensure_slack(self, connector: Any, retries: int,
                      backoff: Callable[[int], float]) -> None:
        """ Ensure we have a SlackClient. """
        connector = self._env_var if connector is None else connector
        slack: SlackClient = _create_slack(connector)
        self._slack = _SlackClientWrapper(
            slack=slack,
            retries=retries,
            backoff=backoff
        )