For Python 3, PEP 3106 changed the design of the dict builtin and the
mapping API in general to replace the separate list based and iterator
based APIs in Python 2 with a merged, memory efficient set and multiset
view based API. This new style of dict iteration was also added to the
Python 2.7 dict type as a new set of iteration methods. PEP-0469 [1]
recommends to replace d.iteritems() to iter(d.items()) to make code
compatible with Python 3.

1. https://www.python.org/dev/peps/pep-0469/

Part of #5538