Update to Django 4.0

[blag]

This PR removes a few features deprecated and removed in Django 4.0, and updates the Travis configuration.

[blag]

It doesn't look like Travis is working. @danfairs You can ask Travis for open source credits (this takes a few hours/days IME, but I think you only need to do it once), or you can migrate over to GitHub Actions.

[julianwachholz]

I have been trying to reach the maintainer but this project seems unmaintained right now. If you need a Django 4 compatible solution now maybe try out django-guest-user

[bittner]

While your PR is good it's mostly for updating the CI setup and string formatting. Which is too much given the original title of the PR. 💣

I fear that is at least one reason why the changes were not yet merged. It should be easy (aka a no-brainer) to merge a suggested change, especially if it's a project the maintainer(s) rarely visit(s). 👀

I'll open a PR that contains the Django 4.0 changes, only. If a miracle happens and that gets merged, it'd be great if you would rebase (or clean up) your changes to have your CI setup contributed. 💯