Adding python version to CI

[kallewesterling]

I wonder if there's a step missing in the ci.yml GitHub action file, where the cache setup is looking for steps.full-python-version.outputs.version, but there is nothing like full-python-version in the steps. I believe, after some googling, that this is the step that should be inserted after Set up Python:

- name: Get full Python version
    id: full-python-version
    run: echo ::set-output name=version::$(python -c "import sys; print('-'.join(str(v) for v in sys.version_info))")

Alternatively, I think we can replace the steps.full-python-version.outputs.version with matrix.python-version as that's what we're running with here?