[UX] Add project_urls to setup.py

[webknjaz]

It'd be great if there were URLs in the PyPI sidebar pointing to this repository. I've got lost trying to navigate here from that page a few times already. PyPI marks GitHub URLs with appropriate icons so adding this would be really helpful.

I know that there are some links in the README that shows up there but I managed to completely miss the right places several times mostly because there's nothing indicating visually that those URLs lead to GitHub.