py3: use next() to obtain next item from inner generator of generatorset .next attribute does not exist on Python 3. As this function seems to really care about the overhead of the Python interpreter, I follow the way of micro optimization.

py3: rewrite itervalues() as values() by importer I'm not a great fan of these importer magics, but this should be okay since "itervalues" seems as unique name as "iteritems".