GitLab

Summary
    Make it get a list of installed interpreters, and add the
    relevant local (HOME) site-packages directory to PYTHONPATH.

    Reason
    To easily add all relevant paths, initially, and between upgrades.

Add check for PYTHONUSERBASE

    Summary
    Check for a non-standard install directory, use it if one
    exists, otherwise use ~/.local.

    Reason
    Allow users to specify their own installation directory, without
    affecting functionality.