Use the python:alpine image for the pip jobs
This image is a mere 12MB to pull and it comes with pip, so let's use that instead of the large Fedora image for jobs where we just run a single script early in the CI.
The first test pipelines show that the check-commit job finishes in under 30s now (over a minute before). And dnf synching is already 100M to download
- time, we're a lot faster with this image.
We only have /bin/sh on this image but the only bash job we were running was the doc build - and that can easily be debashified.