code: def fetch_access_token(self):
          
          expires_in = 7200
          result = self.component.refresh_authorizer_token(
              self.appid, self.refresh_token)
          if 'expires_in' in result:
              expires_in = result['expires_in']
          self.session.set(
              self.access_token_key,
              result['authorizer_access_token'],
              expires_in
          )
          self.expires_at = int(time.time()) + expires_in
          return result
docs: 获取 access token
          详情请参考 https://open.weixin.qq.com/cgi-bin/showdocument?action=dir_list\
          &t=resource/res_list&verify=1&id=open1419318587&token=&lang=zh_CN
  
          这是内部刷新机制。请不要完全依赖！
          因为有可能在缓存期间没有对此公众号的操作，造成refresh_token失效。
  
          :return: 返回的 JSON 数据包