role:python_venv: Implement updates

[NavidSassan]

• weekly updates of pip, wheel, and the "main" package of that venv
• update-strategy eager, but configurable (https://pip.pypa.io/en/stable/development/architecture/upgrade-options/)
• send a mail with "updated ..."
• maybe list the other outdated packages in the mail

there should be one script and config for all the venvs:

• /opt/python-venv/python-venv-update
• /opt/python-venv/python-venv-update.conf

[NavidSassan]

also document this in the README