docs: def _sign_payload(self, payload):
          """使用 appkey 对 payload 进行签名，返回新的请求参数
          """
          app_key = self._app_key
          t = int(time.time() * 1000)
          requestStr = {
              'header': self._req_header,
              'model': payload
          }
          data = json.dumps({'requestStr': json.dumps(requestStr)})
          data_str = '{}&{}&{}&{}'.format(self._req_token, t, app_key, data)
          sign = hashlib.md5(data_str.encode('utf-8')).hexdigest()
          params = {
              't': t,
              'appKey': app_key,
              'sign': sign,
              'data': data,
          }
          return params