[lint] Fix types used for __slots__ and __all__

Stay consistent with the rest of the library and use lists for module
__all__ values and tuples for class __slots__ attributes.

discord/user.py

@@ -466,7 +466,7 @@ class User(BaseUser, discord.abc.Messageable):
         Specifies if the user is a bot account.
     """
 
-    __slots__ = ('__weakref__')
+    __slots__ = ('__weakref__',)
 
     def __repr__(self):
         return '<User id={0.id} name={0.name!r} discriminator={0.discriminator!r} bot={0.bot}>'.format(self)