Fix running on Django master.
Fix running on Django master.

This fixes an exception that was introduced upstream in https://github.com/django/django/commit/226ebb17290b604ef29e82fb5c1fbac3594ac163#diff-ec2bed07bb264cb95a80f08d71a47c06R163-R170

jazzband/django-configurations,jezdez/django-configurations,jazzband/django-configurations