build: fail builds which attempt to install multiple versions of py

[matthugs]

It doesn't make sense and erodes confidence that these services will work without python 3 prior to 3.8 installed

Configuration Pull Request

Make sure that the following steps are done before merging:

• [ ] A SRE team member has approved the PR if it is code shared across multiple services and you don't own all of the services.
• [ ] Are you adding any new default values that need to be overridden when this change goes live? If so:
  • [ ] Update the appropriate internal repo (be sure to update for all our environments)
  • [ ] If you are updating a secure value rather than an internal one, file a SRE ticket with details.
  • [ ] Add an entry to the CHANGELOG.
• [ ] If you are making a complicated change, have you performed the proper testing specified on the Ops Ansible Testing Checklist? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).
• [ ] Think about how this change will affect Open edX operators. Have you updated the wiki page for the next Open edX release?