Update setup.py so we only require ordereddict for Python2.

There's a requirement now on ordereddict, but ordereddict is only relevant for Python2 according to the PyPy page. If it's an install requirement for Python3 in setup.py it messes up making Debian / Ubuntu packages using the standard packaging tools.

This is just a very simple patch so that ordereddict is only required for Python with major version < 3.