Modernize code to use f-strings

This also removes the encoding on the top, since Python 3 does it by
default. It also changes some methods to use `yield from`.

examples/new_member.py

@@ -12,7 +12,7 @@ class MyClient(discord.Client):
     async def on_member_join(self, member):
         guild = member.guild
         if guild.system_channel is not None:
-            to_send = 'Welcome {0.mention} to {1.name}!'.format(member, guild)
+            to_send = f'Welcome {member.mention} to {guild.name}!'
             await guild.system_channel.send(to_send)