Start to add tests

[AlecThomson]

Adds simple tests as a base for more. Going under the pytest framework.

I've also added some dev tools which can be installed like:

pip install ".[dev]"

I'd recommend using this as the base for getting some CI/CD setup. I like GitHub actions, but there are a few options. It'd be good to start getting code coverage reports at the same time. I recently got a nice setup for this over in another repo, if you'd like an example. Check out the config here:
https://github.com/AlecThomson/FixMS/blob/main/.github/workflows/test.yml

[gmloose]

Looks good to me. For testing, you might want to consider using tox.
At ASTRON there's a CI/CD working group that tries to implement good practices w.r.t. to CI/CD. See https://git.astron.nl/templates for more details.

[AlecThomson]

Happy for whatever set of CI/CD tools the maintainers would like to use. And my understanding is that Tox is completely compatible with Pytest tests as well. If its ok, I'd prefer to leave that config to the core maintainers, so that you can decide what platforms you'd like to support :)