python subdirectories

[Gribouillis]

is there a way to have Python make use of yet-another (3rd) such subdirectory name

Yes there are ways. Look for a file named /usr/lib/python3.5/sitecustomize.py or perhaps /etc/python3.5/sitecustomize.py

In this file add this

import site
site.addsitedir('/path/to/your/favorite/directory')

Then you can use /path/to/your/favorite/directory to add your own modules.

It can also be done on a per-user basis. For this create a file $HOME/.local/lib/python3.5/site-packages/usercustomize.py

In this file, write the same thing as above. You can install there scripts for your user only.