add GitHub URL for PyP

[andriyor]

Fixes #

Why is this needed?

Warehouse now uses the project_urls provided to display links in the sidebar on this screen, as well as including them in API responses to help automation tool find the source code for Requests.

Docs: packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/#project-urls

Proposed Changes

• change 1
• change 2
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Does this PR introduce any breaking change?

[andriyor]

url already pointing to readthedocs

[StephenSorriaux]

Thanks for your time on this, I'm closing this since it was finally added in #666