Feature/json session serializer and Django 5 support

[mikemanger]

This is a rebase of #20. The changes are:

• Adds support for Django's default JSONSerializer
• Adds some constants for hardcoded strings
• Support for Django 5 and Python 3.12 to fix Django 5 support #34

As noted in the original PR after changing the serializer you need to clear your sessions with python manage.py clearsessions.

[mikemanger]

One thing I'm confused about is that there are some scary deprecation warnings in 4.1 and 4.2 about some of the templates:

password_policies/tests/test_views.py::PasswordChangeViewsTestCase::test_password_change_confirm
RemovedInDjango50Warning: The "default.html" templates for forms and formsets will be removed. These were proxies to the equivalent "table.html" templates, but the new "div.html" templates will be the default from Django 5.0. Transitional renderers are provided to allow you to opt-in to the new output style now. See https://docs.djangoproject.com/en/4.2/releases/4.1/ for more details

I am struggling to see what is causing this as we override the templates anyway.🤔

EDIT: this was happening before the patch but it might still be relevant for 5.0 support.

[mpasternak]

Commited, thanks! This is a lot of work, thank you!