use gh actions for testing

[s-weigand]

Give a brief summary of the changes.

This PR switches the CI/CD from travis-ci.org and appveyor.org, to GitHub Actions.

Testing

Tests still fail, due to a flake8 issue and an also locally broken test. With those 2 issues fixed, pyglotaran managed to pass the test suite.

TODO

Sadly for the moment we will lose some convenience services:

• coveralls: Atm the normal clients don't support actions and the action doesn't support the coverage output of pytest-cov (see).
• Slack integration: The there are already quite some slack action,
  I need to figure out which one to use and how to configure it. @joernweissenborn I also need access to the secret slack token.
• Deployment: The deployment is added to the actions, but for it to work @jsnel or I need to add our PyPi token to the secrets of the glotaran/pyglotaran repo.
• Deactivate good old travis and appveyor

Closing issues

closes #245