Set supported django version to >= 4.2, <5.1
Drop all currently unsupported django versions (3.2, 4.0, 4.1). Add support for django 5.0.
Run tests against django 5.0 only for Python >= 3.10.
Currently blocked by django-mailman3!239 (merged)
Signed-off-by: David Runge dave@sleepmap.de