plugins/python/example_policy_plugin.py: make allowed_commands ordered

Storing them as "tuple" instead of "set", so they have a fix order.
This makes the output of the list() example stable. ("set" is printed out
in random order)

plugins/python/example_policy_plugin.py

@@ -41,7 +41,7 @@ class SudoPolicyPlugin(sudo.Plugin):
     not acceptable, catch it.
     """
 
-    _allowed_commands = {"id", "whoami"}
+    _allowed_commands = ("id", "whoami")
     _safe_password = "12345"
 
     # -- Plugin API functions --