Ub20: python-pathlib automatically installs python-is-python2

The package apt/python-pathlib (required by armarx/core/arviz_godot) automatically installes python-is-python2. This is bad, as I want to use python3 by default. Further, python-is-python2 forces caffee to use python2 which causes errors during the build process of caffee (libboost-python requires python3).