add comment to explain why it's necessary to do an explicit check for

Python2 and how metaclasses are defined

tests/base/models.py

@@ -16,6 +16,8 @@ class CustomBaseModel(models.base.ModelBase):
 class MyModel(six.with_metaclass(CustomBaseModel, models.Model)):
     """Model subclass with a custom base using six.with_metaclass."""
 
+# This is done to ensure that for Python2 only, defining metaclasses
+# still does not fail to create the model.
 
 if not six.PY3:
     class MyModel(models.Model):