We need to bump the version of django-dynamic-fixture to get the test
suite to run under Python 3. They've dropped Python 2.6 support in the
same release.
Previously, the test suite ran with a custom user model for Django >=
1.5, but it didn't subclass Oscar's AbstractUser. This commit adjusts
the custom user model to subclass Oscar's so we can test functionality
within the custom user class.
This does require a change to the way tests authenticate as we don't
actually use the username field.