Add docs and tentative support mentions for Python 3.11 & Django 4.1

[thibaudcolas]

Description

We’re already testing against Django’s main (currently 4.1 dev). This adds tests for Python 3.11 (alpha release), and updates our support targets to match.

I’ve also removed tests & support mentions for Django 3.0 and 3.1, to reflect the fact they’re now end-of-life.

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

Proposed CHANGELOG entries:

## Added

- Tentatively add support for Python 3.11
- Tentatively add support for Django 4.1

## Removed

- Remove support for Django 3.0.
- Remove support for Django 3.1

[thibaudcolas]

Merging this now without further review as there’s no application change – it’s just metadata, CI setup, docs.