vlc-debian-*, vlc-ubuntu-*: add python3-venv

Needed to integrate python dependencies into the contrib scripts. Unfortunately, despite this functionality being available out of the box on vanilla python, this has been split off into python3-venv on debian/ubuntu-based distros.

It's worth noting that this commit, once the contribs have been updated to include the corresponding python dependencies, will make some of the other python packages we currently install redundant (e.g. python3-mako, and possibly python3-requests).