use python3 for gate

Change-Id: Idc39d6f6e367d3dea673d48a160b8314a51394e8

.zuul.yaml

@@ -23,6 +23,7 @@
         MYSQL_ROOT_PW: secretdatabase
         MISTRAL_USE_MOD_WSGI: true
         MISTRAL_RPC_IMPLEMENTATION: oslo
+        USE_PYTHON3: true
       devstack_services:
         heat: true
         h-api: true

mistral_tempest_tests/tests/api/v2/test_action_executions.py

@@ -95,7 +95,7 @@ class ActionExecutionTestsV2(base.TestCase):
         self.assertIn("output", action_execution)
 
         resp, body = self.client.get_action_execution(action_execution['id'])
-        self.assertIn("output", body)
+        self.assertIn("output", body.decode('utf-8'))
 
         # Test when passing task execution ID