Freshen the CI configuration to make use of our common CI pipeline for
Python projects.

Update the CI configuration to test the range of Python versions which
we claim to support.

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