Fix setup.py so the package can be published to PyPi

[saxbophone]

It turns out that PyPi has changed the package upload process by quite a lot since I last started writing this project. My setup.py script now doesn't work at all!

There is a guide available on the PyPi project for how to migrate older projects, so I should read this and apply which parts of that are relevant to my own package-publishing process.

Note: should use test PyPi to check this is working properly.