Replace six.iteritems() with .items()

1.As mentioned in [1], we should avoid using
six.iteritems to achieve iterators. We can
use dict.items instead, as it will return
iterators in PY3 as well. And dict.items/keys
will more readable. 2.In py2, the performance
about list should be negligible, see the link [2].
[1] https://wiki.openstack.org/wiki/Python3
[2] http://lists.openstack.org/pipermail/openstack-dev/2015-June/066391.html

Change-Id: Ieb7b8a62ef5b8fae79fa41861ad3910ecad988c8

oslo_middleware/debug.py

@@ -19,7 +19,6 @@ from __future__ import print_function
 
 import sys
 
-import six
 import webob.dec
 
 from oslo_middleware import base
@@ -41,7 +40,7 @@ class Debug(base.ConfigurableMiddleware):
         resp = req.get_response(self.application)
 
         print(("*" * 40) + " RESPONSE HEADERS")
-        for (key, value) in six.iteritems(resp.headers):
+        for (key, value) in resp.headers.items():
             print(key, "=", value)
         print()