Script a generator / converter from tox.ini to github

[masklinn]

The ci.yaml job description largely duplicates tox. This means very time CI parameters need to be updated, both have to be separately updated but should match. Ideally, ci.yaml should be partially generated / updated directly from tox.ini (and there should be a job checking they match, probably).

Note that ci.yaml does add features which currently don't exist in tox:

• build wheel and sdist explicitely and cache them
• run tests from wheel, sdist, and source (tox currently only runs tests from wheels)

It's also not entirely clear why tox just installs pyyaml without any further consideration while ci.yaml has to mess about with cyaml, I should probably look back into that.