feat: freshen CI

Freshen the CI configuration to make use of our common CI pipeline for Python projects. Update the tox configuration to actually test the range of Python versions we claim to support in the README which now drops no-longer supported Python versions.

Take the opportunity to move to poetry which allows for dependency pinning for testing. For installation, we use the same version requirements as in the old requirements.txt.

The addition of pre-commit necessitated some (automatic) formatting fixes.

Unfortunately release automation cannot (yet) be enabled because there is not a GitLab token secret configured for this project.

Closes #8 (closed)

Closes #6 (closed)

Closes #2 (closed)