Fix the hacking errors present since neutron-lib 3.21.1

This new neutron-lib 3.21.1 version includes several new hacking checks.
Because the eventlet removal is not finished, the N535 is skipped.

Signed-off-by: Rodolfo Alonso Hernandez <ralonsoh@redhat.com>
Change-Id: Iaad3e87874838eb75d2f6789e71030eda8c587a0
diff --git a/neutron_tempest_plugin/scenario/base.py b/neutron_tempest_plugin/scenario/base.py
index d299c47..f7d08eb 100644
--- a/neutron_tempest_plugin/scenario/base.py
+++ b/neutron_tempest_plugin/scenario/base.py
@@ -18,6 +18,7 @@
 
 from debtcollector import removals
 import netaddr
+from neutron_lib._i18n import _
 from neutron_lib.api import validators
 from neutron_lib import constants as neutron_lib_constants
 from oslo_log import log
@@ -680,8 +681,8 @@
             router = client.update_router(router['id'], **kwargs)['router']
             return router
         else:
-            raise Exception("Neither of 'public_router_id' or "
-                            "'public_network_id' has been defined.")
+            raise Exception(_("Neither of 'public_router_id' or "
+                              "'public_network_id' has been defined."))
 
     def _update_router_admin_state(self, router, admin_state_up):
         kwargs = dict(admin_state_up=admin_state_up)