Add package_url for changelog

Once merged, this PR will show a "Changelog" entry similar to the one [here](https://pypi.org/project/ioc-finder/).

Author: fhightower

setup.cfg

@@ -4,6 +4,7 @@ url = https://github.com/python-jsonschema/jsonschema
 project_urls =
     Funding = https://github.com/sponsors/Julian
     Tidelift = https://tidelift.com/subscription/pkg/pypi-jsonschema?utm_source=pypi-jsonschema&utm_medium=referral&utm_campaign=pypi-link
+    Changelog = https://github.com/python-jsonschema/jsonschema/blob/main/CHANGELOG.rst
     Documentation = https://python-jsonschema.readthedocs.io/en/latest/
     Source = https://github.com/python-jsonschema/jsonschema
     Issues = https://github.com/python-jsonschema/jsonschema/issues/