This adds basic support for Django 4.1. There are no incompatibilities

noticed in 4.1, but there were some changes that affected unit tests.
Tests have been updated to pass across all supported versions of Django,

and both Django 4.1 and Python 3.11 environments were added to tox.
Package metadata now explicitly supports 4.1 and 3.11.

Unit tests pass for all supported versions of Django and Python.