REF/FIX: fix py39-lxml + move packages to variables

[klauer]

• Last-minute touch-ups after I found that py39-lxml was removed without notice from the Beckhoff 13 stable package server
  • Packages are listed here: https://tcbsd.beckhoff.com/TCBSD/13/stable/packages/packagesite.html
  • py39-lxml is in testing, so it may return: https://tcbsd.beckhoff.com/TCBSD/13/testing/packages/packagesite.html
  • py38-lxml is in V12 but I don't want to revert this
• So, in my opinion, the best path forward here is enabling the OpenBSD repositories as a fallback for when TwinCAT repos don't have the packages
  • This is done by removing a stub file that Beckhoff installs specifically disabling the OpenBSD repo
• Additionally, this moves packages to be variables instead of baked into the playbook
  • This just seems to be good or at least better practice
  • I think that "normal" playbooks may have these dependencies baked into individual roles
• Roles are not really in use in this repository - and that may be something to follow up on for actual production deployments
• Adds optional step to install pip packages (and then subsequently remove pip)

[tangkong]

I think this makes sense. Diff seems to match the intention, but I'm no expert. Glad we were able to get lxml elsewhere. I think this ends up needing to be "Python Dev", with an eventual promotion to "Python Library"