Add release notes to project URLs in PyPI (#679)

It's useful to be able to navigate to the release notes easily
from the package index when upgrading.

"Release Notes" is a special keyword that will have the scroll icon
in the project page.

A random example:

* https://pypi.org/project/streamlit/
* https://github.com/streamlit/streamlit/blob/815a3ea6fa3e7f9099b479e8365bd3a5874ddc35/lib/setup.py#L111

Co-authored-by: Lahiru Maramba <llahiru@gmail.com>

Author: 2 people

setup.py

@@ -52,6 +52,7 @@
     long_description=long_description,
     url=about['__url__'],
     project_urls={
+        'Release Notes': 'https://firebase.google.com/support/release-notes/admin/python',
         'Source': 'https://github.com/firebase/firebase-admin-python',
     },
     author=about['__author__'],