add GitHub URL for PyPi
Warehouse now uses the project_urls provided to display links in the sidebar on [this screen](https://pypi.org/project/requests/), as well as including them in API responses to help the automation tool find the source code for Requests.

Docs: [packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#project-urls](https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#project-urls)