Fix dict iteration in Python 3

[sandrinr]

The custom code in "swigify_ib.i" makes use of "list.iteritems()". That method was renamed to "list.items()" in Python 3. To ensure the custom code works in Python 2.7 and 3.x I added the "six" library as a dependency. It enables us to iterate over lists in Python 2.7 and Python 3.x.

[sandrinr]

BTW: This should solve issue #25.

[Komnomnomnom]

Thanks @sandrinr appreciate you taking the time to put together the pull request.

I'd rather not add a dependency on six though. Just the changing the two affected lines to items() should do fine.

[Komnomnomnom]

Closed by c142d2f

[sandrinr]

As far as I understand it swigibpy will not be able to run on Python-2.7 anymore if you just take over the change to list.items(). Since I am not using Python-2 anymore this is acceptable for me.

EDIT: My commend above is actually wrong. list.items() exists also in Python-2.7 but it just has a different memory behaviour than list.iteritems(). So with the patch applied as it is now it should not be a problem.

[Komnomnomnom]

Yep in Python 2 items() returns a list and iteritems() returns an iterator. In Python 3 items() and many other methods (e.g. range) were changed to return iterators and their iterator equivalents were removed. Changes these two lines to use items() has negligible overhead in Python 2.