changed .iteritems() to .items() to fix problems

this may be related to my using rpy2 version 2.4.4 instead of 2.3.8 - but changing this fixed my issue and allowed tests to pass. Apparently a ListVector in rpy2 v 2.4.4. does not have iteritems capability, but does have items - which should work the same. 

I checked rpy2 source and quickly found myself following a chain of parent classes ... but the only items() method (in Vector) was actually an iterator, fwiw.