Use github workflows to test software in different environemnts

[paola-g]

We can use the github framework to test the code with different python versions and across operating systems to enrich the documentation.

[jsheunis]

Using a github action with a "matrix strategy" could be useful: https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/running-variations-of-jobs-in-a-workflow#about-matrix-strategies

Also, I would suggest running the workflow on PRs to master and also on merges to master.

Note: the current documentation build workflow could also be updated to run on PRs (if any content in the docs directory changed in the PR)