Use six.iteritems to make dict work on Python2/3
Use six.iteritems to make dict work on Python2/3

six.iteritems returns an iterator over dictionary‘s items.
This replaces dictionary.iteritems() on Python 2 and dictionary.items()
on Python 3. See http://pythonhosted.org/six/#six.iteritems

Implements: blueprint make-python3-compatible
Change-Id: I90be87979ebc7eccd11d66cd1eef16d9b2eb9fda

cybertron/oslo.serialization,varunarya10/oslo.serialization,openstack/oslo.serialization