apps: force DateTimeField timezone to UTC (#46038)

DRF 3.7 changed the default timezone used by the DateTimeField
serializer field for the default timezone of Django instead of UTC. To
keep GLC plugin retrocompatible we force it to UTC again.

References:
https://www.django-rest-framework.org/community/release-notes/#370
https://github.com/encode/django-rest-framework/pull/5435

src/authentic2_cut/apps.py

@@ -22,7 +22,7 @@ import django.apps
 
 from django.conf import settings
 from django.db import router, DEFAULT_DB_ALIAS
-from django.utils.timezone import now
+from django.utils.timezone import now, utc
 
 from django.urls import reverse_lazy
 
@@ -364,6 +364,9 @@ class AppConfig(django.apps.AppConfig):
                 return data
             serializer.validate = new_validate
 
+            serializer.fields['modified'].timezone = utc
+            serializer.fields['date_joined'].timezone = utc
+
 
     # execute after other modifiers
     a2_hook_api_modify_serializer.order = 999

tests/test_api.py

@@ -30,6 +30,8 @@ def john(glc_app):
     assert response.json['first_name'] == JOHN
     assert response.json['last_name'] == DOE
     assert response.json['email'] == EMAIL
+    assert response.json['date_joined'].endswith('Z')
+    assert response.json['modified'].endswith('Z')
     assert user.first_name == JOHN
     assert user.last_name == DOE
     assert user.email == EMAIL