Multiplatform CI testing

The paper states:

It is cross-platform and has been tested in Linux, Windows, and Mac environments.

Can we work to improve the CI in .gitlab-ci.yml to test across Python versions and supported platforms as a matrix? I'm very experienced doing this on GitHub Actions and would be happy to help, but I'm new to GitLab CI

per https://github.com/openjournals/joss-reviews/issues/4569