WIP: gitlab-sync: Make it a better Python citizen

No intended functional change, but the Docker / testing situation is kinda insane to me. With this, I can just pip install . and run tox on whatever machine I want. I'd like to move further away from relying so much on running inside containers. They make development super awkward, so the tests stage is again being reused, and containers are just for what they're good at deployment, not development environments (imho).