title: How to get a mentioned user's avatar URL with discord.py?
body: <p>How can I get a mentioned user's avatar?<br>
  I can get a mentioned user's ID, but I can't find out how to use it like <code>message.author.avatar_url</code>.<br>
  Can I make this into <code>(Userid).author.avatar_url</code>?</p>
  
  <p>I already got the mentioned user's ID by slicing the message's content.</p>