Fix subunit describe calls for py3

iteritems does not exists in python3, and it's
actually enough to iterate over keys.

Change-Id: I1a614942e25af26fedc720e5ebf8f800e0c54138
diff --git a/tempest/cmd/subunit_describe_calls.py b/tempest/cmd/subunit_describe_calls.py
index 0f868a9..8ee3055 100644
--- a/tempest/cmd/subunit_describe_calls.py
+++ b/tempest/cmd/subunit_describe_calls.py
@@ -294,7 +294,8 @@
             outfile.write(json.dumps(url_parser.test_logs))
         return
 
-    for test_name, items in url_parser.test_logs.iteritems():
+    for test_name in url_parser.test_logs:
+        items = url_parser.test_logs[test_name]
         sys.stdout.write('{0}\n'.format(test_name))
         if not items:
             sys.stdout.write('\n')