Add django 4.1 support

mailman_web/urls.py: Replace the use of django.conf.urls.url with django.urls.path to be django >= 4.0 compatible (django > 3.0 deprecated django.conf.urls.url in favor of django.urls.path, django >= 4.0 removed django.conf.urls.url).

setup.cfg: Change dependency on django to reflect currently supported upstream versions >= 3.2 < 4.2.