Annotate mem_listener_client attribute

... this enables type checking and code completion.

Change-Id: I331ef1379eed1e8882203af2ee8f9719ee7f4224
diff --git a/octavia_tempest_plugin/tests/act_stdby_scenario/v2/test_active_standby.py b/octavia_tempest_plugin/tests/act_stdby_scenario/v2/test_active_standby.py
index 11b0cec..e3f6338 100644
--- a/octavia_tempest_plugin/tests/act_stdby_scenario/v2/test_active_standby.py
+++ b/octavia_tempest_plugin/tests/act_stdby_scenario/v2/test_active_standby.py
@@ -24,6 +24,7 @@
 from tempest.lib import exceptions
 
 from octavia_tempest_plugin.common import constants as const
+from octavia_tempest_plugin.services.load_balancer import v2
 from octavia_tempest_plugin.tests import test_base
 from octavia_tempest_plugin.tests import waiters
 
@@ -35,6 +36,7 @@
     CONF.validation.run_validation,
     'Active-Standby tests will not work without run_validation enabled.')
 class ActiveStandbyScenarioTest(test_base.LoadBalancerBaseTestWithCompute):
+    mem_listener_client: v2.ListenerClient
 
     @classmethod
     def resource_setup(cls):