add GitHub URL for PyPi

[andriyor]

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 the automation tool find the source code for Requests.

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

Hey there! So you want to contribute to a Firebase SDK?
Before you file this pull request, please read these guidelines:

Discussion

• Read the contribution guidelines (CONTRIBUTING.md).
• If this has been discussed in an issue, make sure to link to the issue here.
  If not, go file an issue about this before creating a pull request to discuss.

Testing

• Make sure all existing tests in the repository pass after your change.
• If you fixed a bug or added a feature, add a new test to cover your code.

API Changes

• At this time we cannot accept changes that affect the public API. If you'd like to help
  us make Firebase APIs better, please propose your change in an issue so that we
  can discuss it together.

[lahirumaramba]

Thanks @andriyor !

@egilmorez FYI. We could also use this to link to Documentation in the future.

[egilmorez]

LG, thanks for sending!