Add tests for other python versions

[freemansw1]

Currently, we only run tests on the latest versions of Python and our dependent libraries. We should probably expand this to test multiple python versions. This should be doable through GitHub Actions, similar to how Metpy has done it.

I'm inclined to have us test the latest minor release for every Python version that we support (we are >=3.6 now). It might also be a good idea to test multiple versions of our libraries, but I see a less clear path forward on that.

@mgrover1 given your immense experience here, I'd be interested in hearing your opinion on this!

[JuliaKukulies]

That sounds like a very good idea @freemansw1 and as you say pretty straightforward to add different python versions.

I have participated in another coding project where we also have done this:
https://github.com/SEE-GEO/pansat/blob/main/.github/workflows/install_and_test.yml
(similar to Metpy).

On the same note, we could also test on operating systems other than Ubuntu, or have we deliberately chosen not to do so?

[freemansw1]

With us accidentally breaking compatibility in #175, I've tagged this to be done in v1.5, which is when we drop support for <3.7.

On the same note, we could also test on operating systems other than Ubuntu, or have we deliberately chosen not to do so?

No particular reason, other than the Dockerfile was a copy/paste of something else I had done. We can add other linux OSes to our test suite, but given that we don't have any compiled code, it may be redundant.

[mgrover1]

Happy to help with this effort!

[w-k-jones]

Resolved with #353