Update usage of datetime.timezone.utc based on feedback from django-upgrade feedback

Author: asset-web

browser/models.py

@@ -292,7 +292,7 @@ def has_failed(self):
         else:
             # Still processing?
             # Assume all processing longer than 12hrs is broken?
-            now = datetime.datetime.utcnow().replace(tzinfo=timezone.utc)
+            now = datetime.datetime.utcnow().replace(tzinfo=datetime.timezone.utc)
             timediff = now - self.started_processing
             if timediff.total_seconds() > (12 * 60 * 60):
                 return True