Setup automated tests

[BenediktBurger]

Automated tests would be good:

• [ ] code tests (running pytest)
• [ ] static code tests (mypy?)
• [ ] calculating test coverage
• [ ] linting (e.g. flake8)

[bilderbuchi]

:+1: In a new project, I would try out https://github.com/astral-sh/ruff as a linter. Integrating coverage.py with pytest will not be a problem.