ci-fairy: don't install black on the base image, use a venv instead

The current black package in pip requires a newer version of packaging than our base image has, see commit ecb37ffe for the error message. That commit worked around this by installing black into the CI image but that's only a partial (and temporary) solution, e.g. pip install --upgrade black still fails with the same error and it's likely we get some similar error in the future for some other package.

Fix this by using a python venv instead so the CI jobs can install whatever they want.

This reverts commit ecb37ffe

cc @bentiss