Deprecated iteritems() method not forward-compatible with Python 3

Formulas iterating over dictionaries should use the `items()` method instead of `iteritems()` because the latter method has been [removed from Python 3](https://docs.python.org/3/library/2to3.html#2to3fixer-dict).  While the `items()` method in Python 2 uses more memory than `iteritems()`, these dictionaries are typically small (e.g., sets of settings from Pillar).
