Remove usage of six

Remove six-library Replace the following items with Python 3 style code.
- six.reraise
- six.assertCountEqual
- six.iteritems
- six.next
- six.itervalues

Change-Id: Id5d7e1e75498af7e0551a52a5b2836339748b064
diff --git a/tempest/lib/common/rest_client.py b/tempest/lib/common/rest_client.py
index 2331b9c..573d64e 100644
--- a/tempest/lib/common/rest_client.py
+++ b/tempest/lib/common/rest_client.py
@@ -24,7 +24,6 @@
 from oslo_log import log as logging
 from oslo_log import versionutils
 from oslo_serialization import jsonutils as json
-import six
 
 from tempest.lib.common import http
 from tempest.lib.common import jsonschema_validator
@@ -509,7 +508,7 @@
             if not hasattr(body, "keys") or len(body.keys()) != 1:
                 return body
             # Just return the "wrapped" element
-            _, first_item = six.next(six.iteritems(body))
+            _, first_item = tuple(body.items())[0]
             if isinstance(first_item, (dict, list)):
                 return first_item
         except (ValueError, IndexError):