Permanently add a directory to PYTHONPATH?
Vous devez add your new directory to the variable d’environnement PYTHONPATH, separated by a colon from previous contents thereof. In any form of Unix, vous pouvez do that in a startup script appropriate to whatever shell you’re using (.profile or whatever, depending on your favorite shell) with a command which, again, depends on the shell in question; in Windows, vous pouvez do it through the system GUI for the purpose.
superuser.com may be a better place to ask further, i.e. for more details if you need specifics about how to enrich an variable d’environnement in your chosen platform and shell, since it’s not really a programming question per se.