Add test infrastructure and release automation

[bittner]

To ensure easy maintenance we should add configuration to run linting, tests and automatic releases (e.g. triggered by a Git tag). This may include:

• tox.ini to run flake8, pylint and potentially pytest using tox
• GitHub Actions configuration to run all Tox targets CI-style (e.g. using tox-gh-actions)

Examples

• tox.ini
• GitHub Actions setup