Replace six.iteritems with dict.items

As [1] mentioned, we should use dict.items instead of six.iteritems.
Let's use dict.items as it should return iterators in PY3 as well.
(As for PY2, the performance about list should be negligible)

[1] https://wiki.openstack.org/wiki/Python3

Change-Id: I77b3a71faa71b5f671daff3415e2ae58289fd3ca
diff --git a/tempest/common/custom_matchers.py b/tempest/common/custom_matchers.py
index 8410541..ed11b21 100644
--- a/tempest/common/custom_matchers.py
+++ b/tempest/common/custom_matchers.py
@@ -14,7 +14,6 @@
 
 import re
 
-import six
 from testtools import helpers
 
 
@@ -217,7 +216,7 @@
     """
 
     def match(self, actual):
-        for key, value in six.iteritems(actual):
+        for key, value in actual.items():
             if key in ('content-length', 'x-account-bytes-used',
                        'x-account-container-count', 'x-account-object-count',
                        'x-container-bytes-used', 'x-container-object-count')\