python3: use six.with_metaclass to declare media metaclass

combo/data/models.py

@@ -39,6 +39,7 @@ from django.dispatch import receiver
 from django.forms import models as model_forms
 from django import forms
 from django import template
+from django.utils import six
 from django.utils.encoding import force_text
 from django.utils.html import strip_tags
 from django.utils.safestring import mark_safe
@@ -462,8 +463,7 @@ class CellMeta(MediaDefiningClass, ModelBase):
     pass
 
 
-class CellBase(models.Model):
-    __metaclass__ = CellMeta
+class CellBase(six.with_metaclass(CellMeta, models.Model)):
 
     page = models.ForeignKey(Page)
     placeholder = models.CharField(max_length=20)