Replace CircleCI with GitHub Actions CI

[justinmayer]

After having experimented in other repositories with CI/CD via GitHub Actions for over a year now, I have come to prefer its interface and built-in integration to CircleCI's offering. I have therefore replaced the latter with the former, including the following changes:

• Instead of only installing the current Django version on a single Python version, the new configuration's matrix ensures that the test suite is run on all supported Python + Django combinations.
• Thanks to AutoPub, new package releases will be automatically published to PyPI when an appropriately-formatted RELEASE.md file lands in the primary branch.
• Speaking of the primary branch, it is now named main. Local clones need to be updated to the new branch name.
• A test for TOTP QR codes was failing on Python 3.8+ due to what appears to be a change in SVG/XML tag attribute ordering. This PR includes a fix for that test.

I'm marking this PR as a draft until Django 3.2 is released, after which I'll add the new version to the test matrix before merging. 🚀

[justinmayer]

Many thanks for reviewing, Rémy! 😁