GREG-94: Making method static

gregui/api/views/invitation.py

@@ -267,7 +267,8 @@ class InvitedGuestView(GenericAPIView):
             # TODO: Send an email to the sponsor?
         return Response(status=status.HTTP_200_OK)
 
-    def _verified_fnr_already_exists(self, person) -> bool:
+    @staticmethod
+    def _verified_fnr_already_exists(person) -> bool:
         try:
             person.identities.get(
                 type=Identity.IdentityType.NORWEGIAN_NATIONAL_ID_NUMBER,