Add Django 6 compatibility and modernise codebase

[voyc-jean]

Hello.

Django 5.1 deprecated the undocumented django.utils.itercompat module. Django 6.0 removed it entirely, which leads to this package breaking when imported.

@stephrdev / @codeinthehole Many components of this project are quite outdated at this point. Is there interest in getting things up to date? If so, would it be best to retain compatibility with EOL versions of Python and Django, or create a version 4.0 which only supports Django 5.2+ / Python 3.10+?