This adds formal support for testing and installing on Python 3.12 and

with Django 4.2.
Python 3.12 requires a newer version of the bundled six in order to

import six.moves, but otherwise didn't require any changes.
Django 4.2 passes all unit tests, so we can begin enabling support for

installing alongside it. It's now in our classifiers along with Python

3.12, and enabled in our Django version range.
Tox has been updated to test these versions.

Unit tests for Django 1.6 through 4.2 pass.
Unit tests for Python 2.7 through 3.12 pass.
Ran combinations of the two, and they pass.