At the beginning of the cycle, unit testing on the following Python
runtimes is required:

* Python 3.6 (default in CentOS 8.0 and OpenSUSE Leap 15.1)
* Python 3.7 (used in integration tests on Ubuntu Bionic)
* Python 3.8 (default in Ubuntu 20.04 Focal)

Python 3.7 testing will be dropped once integration tests have moved to
Focal from Bionic. (This will require a separate project-wide goal.)

Change-Id: Ia787cd430c66658ecee6d3f4039d5949e2cb795e