releasetools: Replace iteritems with items.

While in theory this could incur memory overhead for Python 2, the
impact is low for the existing use cases (plus we're moving away from
Python 2).

Bug: 131631303
Test: No additional occurrence of iteritems.
Test: Build with Python 3.
Change-Id: I0205c9edf25f46e3d85967c7dd2c1af035757741