calendar utils: Update to new JSONField

calendar_utils/models.py

 import arrow
 import requests
-from django.contrib.postgres.fields import JSONField
 from django.core.serializers.json import DjangoJSONEncoder
 from django.db import models
 
@@ -13,13 +12,14 @@ def _convert_arrow_to_datetime(event):
     return event
 
 
-class EventsJSONField(JSONField):
+class EventsJSONField(models.JSONField):
     """
     JSONField for lists of events which converts `begin` and `end` to datetime
     on load from DB.
     """
 
     def from_db_value(self, value, expression, connection):
+        value = super().from_db_value(value, expression, connection)
         if value:
             for event in value:
                 event["begin"] = arrow.get(event["begin"]).datetime