WeChat.ServerMessage.XmlMessage.reply_msg

You're seeing just the function reply_msg, go back to WeChat.ServerMessage.XmlMessage module for more information.
Link to this function

reply_msg(signature, nonce, timestamp, encrypt_content)

回包加密 - 官方文档

<xml>
  <Encrypt><![CDATA[encrypt_content]]></Encrypt>
  <MsgSignature><![CDATA[signature]]></MsgSignature>
  <TimeStamp>timestamp</TimeStamp>
  <Nonce><![CDATA[nonce]]></Nonce>
</xml>