Django 3.0 and higher do not include django.utils.six, since they've

moved entirely onto Python 3. In order to provide compatibility going

forward, Django Evolution now bundles its own copy of six as

django_evolution.compat.six, and all imports have been updated

accordingly.

Unit tests pass for all supported versions of Python and Django

(1.6 through 2.2 — more work is required for 3.0 support).