[enh] Use add instead of update

Co-authored-by: Alexandre Aubin <4533074+alexAubin@users.noreply.github.com>

src/user.py

@@ -1343,7 +1343,7 @@ def user_group_update(
 
         if new_attr_dict["mail"]:
             new_attr_dict["objectClass"] = set(group["objectClass"])
-            new_attr_dict["objectClass"].update({"mailGroup"})
+            new_attr_dict["objectClass"].add("mailGroup")
         else:
             new_attr_dict["objectClass"] = set(group["objectClass"]) - {"mailGroup", "mailAccount"}