add tox to test multiple configurations

[klinga]

While basic python versions are tested by Travis, that is not the case for different dependency configurations. This will become more and more important with time. tox will work well with poetry instead of setup.py (reminder: review dependencies config in pyproject.toml file and make sure no unbound dependencies are defined).
This new workflow will change workflow for publishing to PyPI (update release-checklist doc).

see for reference:
https://wrongsideofmemphis.com/2018/10/28/package-and-deploy-a-python-module-in-pypi-with-poetry-tox-and-travis/
https://python-poetry.org/docs/faq/#is-tox-supported