Add support for Django 4.0

[thibaudcolas]

Description

Depends on #163. Updates the project’s test suite and metadata to support Django 4.0. I’ve done some brief testing of the pattern library with this version and couldn’t spot anything off (aside from #166, but I believe that can be done separately).

We could also remove our testing for Django 3.0 and 3.1 at the same time since they’re not technically supported anymore.

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have added an appropriate CHANGELOG entry

---

Suggested CHANGELOG entry:

## Added

- Support Django 4.0 ([#164](https://github.com/torchbox/django-pattern-library/pull/164)).

[thibaudcolas]

Thank you @bcdickinson, I assume you’ve reviewed #163 which this depends on so will merge both now.