The iteritems() method of Python 2 dictionaries was renamed to items()
on Python 3. The overhead of creating a temporary list for items() on
Python 2 is negligible.
Replace also dict.itervalues() with six.itervalues(dict) in
horizon/test/utils.py.
Partial-Implements: blueprint porting-python3
Change-Id: Ib2d62236be2620c4626099ce80e6c8a9397a4533