Add funding URL to setup.cfg

Author: tlinhart

setup.cfg

@@ -9,6 +9,7 @@ author_email     = [email protected]
 url              = https://jupyter.org
 project_urls     =
     Documentation = https://jupyter-core.readthedocs.io/
+    Funding       = https://numfocus.org/
     Source        = https://github.com/jupyter/jupyter_core
     Tracker       = https://github.com/jupyter/jupyter_core/issues
 license          = BSD-3-Clause