CI: basic GitHub actions

[mion00]

This is a building block for #1652. It provides a simple setup for linting the Python code and running unit tests on every Git push and PR.

Description

• Add Github Actions workflow that gets triggered on every Git push (inspired by official Github examples):
  • the pipeline run against multiple python versions: "3.7", "3.8", "3.9", "3.10" (as does the current azure-pipeline config)
  • install dependencies from requirements.txt and cache them for faster builds
  • lint the code using flake8
  • run unit tests using pytest
• Add tests/test_main.py with sample unit test written for pytest framework (otherwise it errors out with No tests found)
• Add pyproject.toml to configure pytest (as from their best practises docs).
• Move blake and flake8 config to pyproject.toml and .flake8 files for better readability. This way it is possible to run both tools locally with the same config that is used in the CI pipeline (useful for troubleshooting possible config or linting mistakes).

[acockburn]

Looks good to me after review. When I merge this should I remove the Azure integration since we don't need both?

[mion00]

Yeah I suppose since we are migrating to Github Action it is ok to disable Azure pipelines, otherwise we have both running at the same time.

[acockburn]

OK, I removed it in the dev branch – lets see how it goes 😊