Properly populate __slots__ for derived user types.

Fixes #2265

discord/user.py

@@ -294,7 +294,8 @@ class ClientUser(BaseUser):
     premium_type: :class:`PremiumType`
         Specifies the type of premium a user has (e.g. Nitro or Nitro Classic). Could be None if the user is not premium.
     """
-    __slots__ = ('email', 'locale', '_flags', 'verified', 'mfa_enabled',
+    __slots__ = BaseUser.__slots__ + \
+                ('email', 'locale', '_flags', 'verified', 'mfa_enabled',
                  'premium', 'premium_type', '_relationships', '__weakref__')
 
     def __init__(self, *, state, data):
@@ -650,7 +651,7 @@ class User(BaseUser, discord.abc.Messageable):
         Specifies if the user is a bot account.
     """
 
-    __slots__ = ('__weakref__',)
+    __slots__ = BaseUser.__slots__ + ('__weakref__',)
 
     def __repr__(self):
         return '<User id={0.id} name={0.name!r} discriminator={0.discriminator!r} bot={0.bot}>'.format(self)