Add Changelog URL to project metadata
Add Changelog URL to project metadata

For easy/"standard" access from the PyPI project page.

`Changelog` matches the title of the linked doc here, and is one of
the recognized names that get the distinctive icon:
https://github.com/pypa/warehouse/blob/64102a6a41f024e54bc0cfe52201d44bc80afd17/warehouse/templates/packaging/detail.html#L24

An example how it looks can be found e.g. in
https://pypi.org/project/black/

jorisroovers/gitlint,jorisroovers/gitlint