ci: add automated testing mechanism

[AngryMaciek]

Since this is a public repo unlimited minutes of GitHub Actions for CI/CD are available!
Assuming that #4 is addressed it would be very nice to set up a simple GH Actions workflow which would trigger a testrun with every commit being pushed or every merge request being issued. You can take a look at an example CI/CD workflow here.

[AngryMaciek]

Since README.md described additional difficulties for running on macOS it would be very beneficial to present a CI with the tests running on a mac machine (availble through GH Actions, too). This would solidify how to setup code execution on that platform.

[AngryMaciek]

For the CI testing, actually, it might be a good idea to introduce a top level requirements.txt file with fixed versions of the dependencies. So that the test never suddenly break because an external package changed behaviour....

[pgrobecker]

Thanks for all of your advice regarding testing, I'll look into how I can make it more systematic and automated