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/raw_models.py

@@ -1,5 +1,3 @@
-# -*- coding: utf-8 -*-
-
 """
 The MIT License (MIT)
 
@@ -26,8 +24,8 @@ DEALINGS IN THE SOFTWARE.
 
 class _RawReprMixin:
     def __repr__(self):
-        value = ' '.join('%s=%r' % (attr, getattr(self, attr)) for attr in self.__slots__)
-        return '<%s %s>' % (self.__class__.__name__, value)
+        value = ' '.join(f'{attr}={getattr(self, attr)!r}' for attr in self.__slots__)
+        return f'<{self.__class__.__name__} {value}>'
 
 class RawMessageDeleteEvent(_RawReprMixin):
     """Represents the event payload for a :func:`on_raw_message_delete` event.