This code does not have any dependencies that are specific to any specific
version of ubuntu.  So instead of testing on a specific version and then needing
to do work to keep the versions up-to-date, we switch to the ubuntu-latest
target which should be sufficient for testing purposes.

This work is being done as a part of openedx/platform-roadmap#377

closes #106

Since the toxenv was not updated, we were previously only running tests
on Python 3.8 even in the 3.11 runs.  This should fix that.