Update to Ansible 2.7 running Python 3.8
Update to Ansible 2.7 running Python 3.8

This port is scheduled to be removed soon. For now, switch everything to
running Ansible 2.7 until everything can be updated to support running
under Ansible 2.9. Note, this change also switches us from Python 2 to
Python 3.

seankelly/buildbot-infra,seankelly/buildbot-infra,seankelly/buildbot-infra,buildbot/buildbot-infra,buildbot/buildbot-infra,buildbot/buildbot-infra