[test] Add user-site to PYTHONPATH before installation of new cube in test_newcube_install Otherwise, setuptools complains that the target installation directory (i.e. "user site" is not in PYTHONPATH). This is probably because we use a fake HOME with --user install option. Failure shows up when running tests through tox without system site packages (see next patch).

raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")