Replaces .next() method calls with function calls.
Replaces .next() method calls with function calls.

This improves (provides, perhaps?) Python 3 compatibility (at least for
3.3+) with prettyplotlib. Python 3 promoted next() to a full function of
the language, and consequently removed the .next() method from
iterables. next() was backported to Python 2.6. See
http://docs.python.org/3/whatsnew/2.6.html#other-language-changes.
Matplotlib has required a minimum Python version of 2.6 since v1.2,
thus, prettyplotlib loses no compatibility.

olgabot/prettyplotlib,olgabot/prettyplotlib