Each of the Python repositories we updated to support Django 2.2 now needs to have its test matrix updated to stop testing Django versions prior to 2.2 and start testing Python 3.8. Update .travis.yml and tox.ini accordingly (ideally using the scripts produced for https://openedx.atlassian.net/browse/BOM-1507 and https://openedx.atlassian.net/browse/BOM-1508), and upgrade any packages that were being held back only because of missing support for one of the removed Django versions. Also, update the Trove classifiers in setup.py to reflect the Python and Django versions currently being tested. For packages already on PyPI, go ahead and make a new release so the Trove classifiers are updated there also.
Do not yet try to fix any test failures under Python 3.8; simply mark them in allow_failures and write a new backlog BOM ticket in https://openedx.atlassian.net/browse/BOM-1521 to fix them later. Similarly, Django 3.0 testing may be added to tox.ini, but do not yet add it to .travis.yml; at this point in time, running those tests is just a distraction from the higher-priority Python upgrade.