A heads-up for anyone else on RHEL 8, because the error is mystifying.
django-mailman 1.3.12 bumps the preferred django version from 4.1 to 4.2. Django 4.2 requires MariaDB 10.4 or later. RHEL 8 has full lifecycle support for MariaDB 10.3.
For some reason I haven't yet been able to identify, this causes pyMySQL integrations to fail with the mysterious error: django.core.exceptions.ImproperlyConfigured: mysqlclient 1.4.3 or newer is required; you have 1.4.6.
(Yes, Database.version_info contains "(1, 4, 2, 'final', 0)" while Database.version contains "1.4.6" and I am totally stumped where that 1.4.2 is coming from as it appears... nowhere.)
Bypassing that error check gives the more informative: django.db.utils.NotSupportedError: MariaDB 10.4 or later is required (found 10.3.39).
Doing a pip install with django\<4.2 works. This is not ideal since 4.1 is past LTS, so I would recommend that Mailman systems be migrated to RHEL 9.
--Jered