Django 4.2 and fix tests

[blag]

Closes #354, but also does more.

Supercedes #369.

This PR is prep for a release. I'd like to get CI passing before I submit a PR to bump the version as part of a release.

Changes proposed in this PR:

• Move accounts/tests to ./tests (so we don't distribute tests with package)
• Use timezone-aware now() in middleware
• Fix tests
• Use ruff
• Drop Django 2.2, and Python 3.6 and 3.7
• Update GHA config

Tips for an ideal PR

• You have read our Code of Conduct: https://github.com/pinax/.github/blob/master/CODE_OF_CONDUCT.md
• You have read our Contributing info: https://github.com/pinax/.github/blob/master/CONTRIBUTING.md
• The PR is atomic
• Unit tests have been updated/added, if needed
• App version number has been updated in setup.py (Pinax uses SemVer)
• Change Log has been updated
• You have added your name to the AUTHORS file

[blag]

Okay, I think I figured out DeepSource. This is ready for review. I'll merge it once I get a signoff. 😄

[uhurusurfa]

🚀 looks good