Python3: use six.iteritems() instead of dict.iteritems()

This also adds a check to neutron/hacking/checks.py that should catch this
error in the future.

Blueprint: neutron-python3
Change-Id: Ie7b833ffa173772d39b85ee3ecaddace18e1274f
diff --git a/neutron/tests/tempest/common/custom_matchers.py b/neutron/tests/tempest/common/custom_matchers.py
index 298a94e..839088c 100644
--- a/neutron/tests/tempest/common/custom_matchers.py
+++ b/neutron/tests/tempest/common/custom_matchers.py
@@ -14,6 +14,7 @@
 
 import re
 
+import six
 from testtools import helpers
 
 
@@ -121,7 +122,7 @@
     """
 
     def match(self, actual):
-        for key, value in actual.iteritems():
+        for key, value in six.iteritems(actual):
             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')\