Fix Python 3, also add test for count across multiple backends

I added the Py3 travis links when removing 2.5 but yes, should have checked that it worked :)

The Python 3 issues were:

- Except syntax (no longer supports the comma version)
- iteritems() has been removed (see e.g.http://stackoverflow.com/questions/13998492/iteritems-in-python)
- I had to change the zip method in the test as it is a generator and not subscriptable