Establish a new session.

def new_session(self):
        """Establish a new session."""
        body = yield from self._fetch_json(URL_LOGIN, self._new_session_data)
        self.sma_sid = jmespath.search('result.sid', body)
        if self.sma_sid:
            return True

        msg = 'Could not start session, %s, got {}'.format(body)

        if body.get('err'):
            if body.get('err') == 503:
                _LOGGER.error("Max amount of sessions reached")
            else:
                _LOGGER.error(msg, body.get('err'))
        else:
            _LOGGER.error(msg, "Session ID expected [result.sid]")
        return False