Add support for animated guild banners

https://github.com/discord/discord-api-docs/pull/3948

discord/asset.py

@@ -208,11 +208,13 @@ class Asset(AssetMixin):
 
     @classmethod
     def _from_guild_image(cls, state, guild_id: int, image: str, path: str) -> Asset:
+        animated = image.startswith('a_')
+        format = 'gif' if animated else 'png'
         return cls(
             state,
-            url=f'{cls.BASE}/{path}/{guild_id}/{image}.png?size=1024',
+            url=f'{cls.BASE}/{path}/{guild_id}/{image}.{format}?size=1024',
             key=image,
-            animated=False,
+            animated=animated,
         )
 
     @classmethod

discord/guild.py

@@ -197,6 +197,7 @@ class Guild(Hashable):
 
         They are currently as follows:
 
+        - ``ANIMATED_BANNER``: Guild can upload an animated banner.
         - ``ANIMATED_ICON``: Guild can upload an animated icon.
         - ``BANNER``: Guild can upload and use a banner. (i.e. :attr:`.banner`)
         - ``COMMERCE``: Guild can sell things using store channels.