ci: Switch from GitHub Actions to GitLab CI
- Remove GitHub Actions
- Add a basic GitLab CI config to run on tox tests using docker.io containers.
- Update config for tox 4.0
- Remove dependency on tox-wikimedia (see T325260)
- Drop Python versions 3.4, 3.5, 3.6
- Add Python version 3.11
- Run mypy with Python version 3.9
- Only run flake8 once with Python version 3.9
Bug: T333504