CLN: Use next function rather than next method.
CLN: Modernize range + xrange.

Now range is always equivalent to 2.X xrange throughout (but need to
import range from py3compat to use it). Also remove range fixer from
setup.py.
CLN: Use Python 2/3 compatible iteritems.

Switches calls to `iteritems` method to use
`pandas.util.compat.iteritems`, which calls `iteritems` if it exists and
otherwise calls `items`.
CLN: iterkv --> iteritems