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: I83997d0c43826c9d5b06a72d93365e8e9b4587af

openstack_releases/yamlutils.py

@@ -38,7 +38,7 @@ class PrettySafeDumper(yaml.dumper.SafeDumper):
             'tag:yaml.org,2002:map', values, flow_style=None)
         if self.alias_key is not None:
             self.represented_objects[self.alias_key] = node
-        for key, value in six.iteritems(data):
+        for key, value in data.items():
             key_item = self.represent_data(key)
             value_item = self.represent_data(value)
             values.append((key_item, value_item))