Refactor CI code to fix current failures with "No module named pytest"

CI started failing with:
/builds/ollieparanoid/pmbootstrap/venv/bin/python3: No module named pytest

I've briefly tried to fix this with the existing scripts. However,
instead of investing more time into that, do the long overdue
refactoring of the scripts that involve dropping the venv logic and
support for a custon gitlab-ci-runner using some python docker image as
base. This configuration hasn't been used for a long time and is
probably broken anyway.

Refactor the logic to skip the qemu test case in gitlab CI by using
pytest markers. The new script is now similar to bpo's .ci/pytest.py.

• other CI refactoring while at it, see commits for details

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Urgent fix, will merge it as soon as CI passes.