[mailman3] Fix the rest API on mailman

The API didn't work and failed with 500, because the REST framework didn't know
how to render the response. This fixes the issue.

https://pagure.io/fedora-infrastructure/issue/12134

Signed-off-by: Michal Konecny <mkonecny@redhat.com>

roles/mailman3/templates/settings.py.j2

@@ -323,6 +323,9 @@ Q_CLUSTER = {
 #
 REST_FRAMEWORK = {
     'PAGE_SIZE': 10,
+    'DEFAULT_RENDERER_CLASSES': [
+        'rest_framework.renderers.JSONRenderer',
+    ],
     'DEFAULT_FILTER_BACKENDS': (
         'rest_framework.filters.OrderingFilter',
     ),