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.

The auth backend now gracefully handles situations where there are users
with the same email address.