[enh] #149: Set-up CI/CD adding .gitlab-ci.yml file

- #245: Use own built images with Poetry
- Checks: build, format
- Use Black installed from Docker image v3.8
- tests: Pytest for Python v3.5, 3.6, 3.7 (+ coverage), and 3.8
- Coverage published to GitLab Pages once tests-3.7 job is done