Bug description:
Some files were installed using absolute path, preventing installation
under non-root user.

Fix description:
Change paths to be relative to the current prefix.
Update .gitignore to exlcude venv and build products.
Update tox.ini to the current supported Python versions.

Fixes https://pagure.io/389-ds-base/issue/50037

Reviewed by: mhonek, firstyear, spichugi (Thanks!)