code: def create_rich_menu(self, rich_menu, timeout=None):
          
          response = self._post(
              '/v2/bot/richmenu', data=rich_menu.as_json_string(), timeout=timeout
          )
  
          return response.json.get('richMenuId')
docs: Call create rich menu API.
  
          https://developers.line.me/en/docs/messaging-api/reference/#create-rich-menu
  
          :param rich_menu: Inquired to create a rich menu object.
          :type rich_menu: T <= :py:class:`linebot.models.rich_menu.RichMenu`
          :param timeout: (optional) How long to wait for the server
              to send data before giving up, as a float,
              or a (connect timeout, read timeout) float tuple.
              Default is self.http_client.timeout
          :type timeout: float | tuple(float, float)
          :rtype: str
          :return: rich menu id