title: discord.py get member instance from name#discriminator
body: <p>I have a string with the name of an user <code>user = 'somename#1111'</code> how can I mention him? (I get the name from google sheets that's why it's a string)</p>
  
  <p>I tried with:</p>
  
  <pre><code>@client.command()
  async def mention(ctx):
      user : discord.Member = 'somename#1111'
      await ctx.send(f"{user.mention} hello!")
  </code></pre>
  
  <p>But it doesn't work</p>