[Python 3] Use .items() instead of .iteritems()
.iteritems() was dropped in Python 3.0.
This commit may have a negative performance impact on Python 2 since
.items() returns a copy of the dictionary's item list. In Python 3,
.items() returns a view object, so no performance drop should be
Reviewers: machniak, mollekopf
Reviewed By: mollekopf
Differential Revision: https://git.kolab.org/D3244