[enh] #149: Add CI/CD .gitlab-ci.yml

- Use our built images
- Checks: build, format
- tests: pytest for Python v3.5, 3.6, 3.7 (+ coverage), and 3.8
- #105: PyPi and PyPi test distributions
- Coverage published to GitLab Pages once tests-3.7 job is done
- Use Black installed from Docker image v3.8 as black is only available on Buster