[paperwork] Add explicit backend to User.with_perms call

paperwork/certification_emails.py

@@ -90,7 +90,11 @@ def admin_email(ordered_queryset):
         to=(
             get_user_model()
             .objects.with_perm(
-                "paperwork.receive_certification_emails", include_superusers=False
+                "paperwork.receive_certification_emails",
+                include_superusers=False,
+                # TODO: backend should not be hardcoded
+                # TODO: LDAPBackend does not support with_perm() directly
+                backend="django.contrib.auth.backends.ModelBackend",
             )
             .values_list("email", flat=True)
         ),