Reawaken development

[stevejalim]

This changeset aims to jump start development on the project.

• Update tox matrix to test against latest/non-EOL Django and Python versions
• Update CI to use GHA instead of CircleCI
• Drop six (resolves Remove package six  #199)
• Swap pep8 and flake8 for ruff
• Add support for pre-commit

Once this is merged to main, we can look at any outstanding PRs and merge those that are safe/OK to merge. We'll then look to do a 3.8alpha or 3.8rc release, before a formal 3.8 release.

The longer goal remains to get the project into a state where it can hopefully be absorbed into Django Core - see #135

[stevejalim]

@tim-schilling I'd welcome your review on this, too, if you don't mind

[tim-schilling]

It looks good to me. I didn't run anything, just gave the code a once over. The issue I spotted wouldn't harm anything even.

[robhudson]

r+wc - just noticed some string clean-up that isn't necessary but a nice-to-have to avoid awkward concatenations, presumably from the automated formatting.