Merge branch 'python3-support' into 'master'

Python3 support

This branch is based on GitLab-CI support, and should be merged after !1.

We added support for both `python2` and `python3` in the codebase, but it still has python2 hard-coded.

For now we are patching the interpreter used in omnibus to change it to python3 to keep backward compatibility with users using it by source.

In the near future, we will make this switchable with the help of an initializer.

See merge request !2