The packages are stuck on py3.6 which means they will be pain once we switch to 3.7. Please test them on 3.7 *and* 3.8 (so we don't to revisit this in a few months), and update PYTHON_COMPAT appropriately. If it doesn't work, please either fix it, remove Python or issue last rites. Please consider this urgent.
FWICS the tests seem to be broken with current marshmallow version (they fail to import marshmallow.compat).