Add changelog URL to package metadata

This changelog link will be visible on the package's PyPI page.

Also Renovate bot uses this link to provide quicker access to the changelog for a dependency update.

setup.cfg

@@ -4,6 +4,7 @@ max-line-length = 110
 [metadata]
 project_urls =
     Source=https://github.com/Legrandin/pycryptodome/
+    Changelog=https://www.pycryptodome.org/src/changelog
 
 [bdist_wheel]
 py-limited-api = cp35