Read a set of keys.

def read(self, sensors):
        """Read a set of keys."""
        payload = {'destDev': [], 'keys': list(set([s.key for s in sensors]))}
        if self.sma_sid is None:
            yield from self.new_session()
            if self.sma_sid is None:
                return False
        body = yield from self._fetch_json(URL_VALUES, payload=payload)

        # On the first 401 error we close the session which will re-login
        if body.get('err') == 401:
            _LOGGER.warning("401 error detected, closing session to force "
                            "another login attempt")
            self.close_session()
            return False

        _LOGGER.debug(json.dumps(body))
        for sen in sensors:
            if sen.extract_value(body):
                _LOGGER.debug("%s\t= %s %s",
                              sen.name, sen.value, sen.unit)
        return True