adding simple GH workflow for testing

[Borda]

It would be great to be sure that any change or addition does not break any existing use case... so adding a simple CI job running with several Python versions between 3.8 and 3.11 for all their major OS

To be done or clarify: so far, it seems that eventually, @onelivesleft all testing is only via visual inspection of running these two scripts?

I run the CI in my fork as draft PR, see: https://github.com/Borda/PrettyErrors/pull/1