Add a PYTHONPATH that is not dependent on the Python version

[probonopd]

We should add a PYTHONPATH that is not dependent on the Python version so that we can install some site-packages there without them breaking whenever the minor version of Python changes.

By default there is just:

FreeBSD% python3 -c "import sys; print('\n'.join(sys.path))"

/usr/local/lib/python38.zip
/usr/local/lib/python3.8
/usr/local/lib/python3.8/lib-dynload
/home/user/.local/lib/python3.8/site-packages
/usr/local/lib/python3.8/site-packages

[probonopd]

In the meantime, we can

import sys
sys.path.append('/usr/local/lib/python3/site-packages')
. . .
import <your-python-module>

[probonopd]

This is what we are doing in https://github.com/helloSystem/ISO/commit/0d712b4e7b762ea521d707ab3e42910f78787398 now.