Add Download link to setup.cfg

[skip ci]

setup.cfg

@@ -35,6 +35,7 @@ classifiers =
 author = Panda3D Team
 author_email = [email protected]
 project_urls =
+    Download=https://www.panda3d.org/download/
     Source=https://github.com/panda3d/panda3d
     Tracker=https://github.com/panda3d/panda3d/issues
     Documentation=https://www.panda3d.org/manual/