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: Ifbe7929580ec61a2cd50232794c0cbd0cbf37edc

oslo_serialization/jsonutils.py

@@ -150,7 +150,7 @@ def to_primitive(value, convert_instances=False, convert_datetime=True,
                                       encoding=encoding)
         if isinstance(value, dict):
             return {recursive(k): recursive(v)
-                    for k, v in six.iteritems(value)}
+                    for k, v in value.items()}
         elif hasattr(value, 'iteritems'):
             return recursive(dict(value.iteritems()), level=level + 1)
         # Python 3 does not have iteritems

oslo_serialization/msgpackutils.py

@@ -172,7 +172,7 @@ class HandlerRegistry(object):
     def copy(self, unfreeze=False):
         """Deep copy the given registry (and its handlers)."""
         c = type(self)()
-        for ident, handlers in six.iteritems(self._handlers):
+        for ident, handlers in self._handlers.items():
             cloned_handlers = []
             for h in handlers:
                 if hasattr(h, 'copy'):