T6329: firewall: use isinstance() in op-mode script

(cherry picked from commit b705adc)

src/op_mode/firewall.py

@@ -531,7 +531,7 @@ def find_references(group_type, group_name):
                             continue
 
                         for idx, member in enumerate(members):
-                            if type(member) == str:
+                            if isinstance(member, str):
                                 # Only member, and no timeout:
                                 val = member
                                 timeout = "N/D"