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

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):