add backlink to main repository

[domanchi]

Problem

Discovery of the underlying package source code was difficult to find from the official pypi page of this module.

Additionally, it was undiscoverable from a Google search soundcloudpy github from my location.

Solution

Add metadata to the Python package bundling to make discoverability a little easier for humans and crawlers alike.

Used https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#urls as a reference.

[OzGav]

@robsonke