Decide python versions we support

Currently, the tests are expected to run on. For tests, currently we'll be using `` 3.8.11-slim-buster`` but if we plan to support more versions, we should run tests for them explicitly.

via: https://github.com/nexB/vulnerablecode/wiki/WeeklyMeetings#meeting-on-tuesday-2021-07-27-at-1300-utc