Return png instead of webp when getting avatar image URL

WebP images have a spotty reputation on iOS with rendering.
Changing this to a PNG lets the avatar URL display correctly on iOS.

disco/types/user.py

@@ -30,7 +30,7 @@ class User(SlottedModel, with_equality('id'), with_hash('id')):
         if self.avatar.startswith('a_'):
             return 'https://cdn.discordapp.com/avatars/{}/{}.gif?size={}'.format(self.id, self.avatar, size)
         else:
-            return 'https://cdn.discordapp.com/avatars/{}/{}.webp?size={}'.format(self.id, self.avatar, size)
+            return 'https://cdn.discordapp.com/avatars/{}/{}.png?size={}'.format(self.id, self.avatar, size)
 
     @property
     def default_avatar(self):