With Travis CI we also ran the test against the behave-latest target. That needs to be added to GHA.
The test matrix spun up by the GHA configuration is the full combination of all Python and Django versions, but some combinations are not supported and hence excluded in the Tox configuration (e.g. Django 4.0 doesn't run on Python 3.6, 3.7, Django 2.2 not on Python 3.10). The related jobs need to be excluded from the matrix.
Travis CI has stopped working for a while. We move to GitHub Actions just like many other projects.
We also move tool configuration settings to pyproject.toml, which seems to be getting more and more popular among Python tool developers.
Closes #128