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`.

discord/role.py

@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-
 """
 The MIT License (MIT)
 
@@ -248,7 +246,7 @@ class Role(Hashable):
     @property
     def mention(self):
         """:class:`str`: Returns a string that allows you to mention a role."""
-        return '<@&%s>' % self.id
+        return f'<@&{self.id}>'
 
     @property
     def members(self):