Add pyproject.toml with pytest configuration to run tests outside of tox

We had configured tox to do some PYTHONPATH magic so that the
application code would be found by the tests, however tox is
causing use more and more headaches, so let's allow running
pytest directly.

Here we add the necessary pytest configuration to set the
pythonpath, just like tox did.

We use pyproject.toml as the new Python standard,
we should eventually move there all of setup.cfg.