Use f-strings in more places that were missed.

discord/sticker.py

@@ -88,7 +88,7 @@ class Sticker(Hashable):
         self.preview_image = data.get('preview_asset')
 
     def __repr__(self):
-        return '<{0.__class__.__name__} id={0.id} name={0.name!r}>'.format(self)
+        return f'<{self.__class__.__name__} id={self.id} name={self.name!r}>'
 
     def __str__(self):
         return self.name