Matrix unit tests with Python version and OS

[timothy-nunn]

With the removal of Fortran and CMake, there is no more build CI job. This should make it possible/easier to run some of our tests as a matrix. I think we should run at least our unit tests on all versions of Python we support and on Ubuntu and Mac.

[timothy-nunn]

If we want to run more tests with a matrix, we must be mindful that some of our CI uses apt to install PDF dependencies which will not work on Mac runners.