I just did a quick test on Debian Stable (Buster), but Seahub didn’t come up:
django.core.exceptions.ImproperlyConfigured: mysqlclient 1.3.13 or newer is required; you have 1.3.10.
Django 2.2 requires python3-mysqldb => 1.3.13, but this package is 1.3.10 on stable.
There are Buster Backports for Django 2.2, so I guess, this can be fixed somehow.