Fixed #19576 -- Use `six.with_metaclass` uniformously accross code base.

django/db/models/base.py

@@ -311,7 +311,7 @@ class ModelState(object):
         self.adding = True
 
 
-class Model(six.with_metaclass(ModelBase, object)):
+class Model(six.with_metaclass(ModelBase)):
     _deferred = False
 
     def __init__(self, *args, **kwargs):

django/db/models/query.py

@@ -990,7 +990,7 @@ class InstanceCheckMeta(type):
     def __instancecheck__(self, instance):
         return instance.query.is_empty()
 
-class EmptyQuerySet(six.with_metaclass(InstanceCheckMeta), object):
+class EmptyQuerySet(six.with_metaclass(InstanceCheckMeta)):
     """
     Marker class usable for checking if a queryset is empty by .none():
         isinstance(qs.none(), EmptyQuerySet) -> True