[tests] integration into gitlab-ci, py3-from-forge and trigger cubicweb builds from logilab-mtconverter if all other tests passed
Continuation of the work on yams!15 (closed) to trigger build on projects on which this one depends (cubicweb here).
Also:
- add a gitlab-ci.yaml based on tox
- add py3-from-forge tox command and add it into gitlab-ci.yaml