Add CI to automatically build current wheels against stable/testing/staging

As we want to ensure that the current integrations builds and installs, we want CI testing this.

This can be as easy as running

python -m bootstrap.build
python -m bootstrap.install dist/*-py3-none-any.whl

Ideally we want this for running against stable, testing(?) and staging repositories, so that we can check our assumptions against an updated Python interpreter in staging.