lisa_shell: Check PYTHONPATH
Warn if PYTHONPATH is not empty, since it can interfere with venv. In the worst case, it can lead to using a wrong version of e.g. bart that would be installed somewhere else, leading to strange failures in LISA.
Warn if PYTHONPATH is not empty, since it can interfere with venv. In the worst case, it can lead to using a wrong version of e.g. bart that would be installed somewhere else, leading to strange failures in LISA.