[Python 3] Use .items() instead of .iteritems()

Summary:
.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
noticeable there.

Reviewers: machniak, mollekopf

Reviewed By: mollekopf

Subscribers: mollekopf

Differential Revision: https://git.kolab.org/D3244