[1.5.x] Fixed #19576 -- Use `six.with_metaclass` uniformously accross code base.
[1.5.x] Fixed #19576 -- Use `six.with_metaclass` uniformously accross code base.

Backport of f58efd07f from master.

bliti/django-nonrel-1.5,imtapps/django-imt-fork,alx-eu/django,hasadna/django,ccn-2m/django,hasadna/django,hasadna/django,bliti/django-nonrel-1.5,bliti/django-nonrel-1.5,alx-eu/django,ccn-2m/django,alx-eu/django,ccn-2m/django,alx-eu/django,imtapps/django-imt-fork,imtapps/django-imt-fork,ccn-2m/django