GitLab

[skip ci]

Always load library builders (for OSX as well as Manylinux).

The thing that sets the Python version for building and testing is the
MB_PYTHON_VERSION variable.

Build / test Python versions conceptual separable in case of Linux
builds.

Make configuration more general.

Move responsibility for source futzing back to the travis.yml file,
because we are now doing only one build per job.