Support more CI systems

This bug is just here to indicate that I'm open to adding support for additional CI systems, especially if people send me pull requests ;)

Things like

- GitLab CI (example: https://gitlab.com/anarcat/undertime/-/blob/master/.gitlab-ci.yml)
- GitHub actions (example: https://github.com/zopefoundation/z3c.jbot/blob/master/.github/workflows/test.yml)
- CircleCI?
- Azure something-or-other?
- what else is there?

Ideally I'd like several examples to see what the config files look like.

Start by reviewing the repository's current handling of setup.py, tox.ini, and .travis.yml, then compare the linked GitLab CI and GitHub Actions examples. The scope needs agreement on which CI systems and configuration patterns to support; done means the selected configurations are recognized and checked consistently.

Written by the indexing model from the issue text.