Please branch and build python-nbformat in epel9. If you do not wish to maintain python-nbformat in epel9, or do not think you will be able to do this in a timely manner, the EPEL Packagers SIG would be happy to be a co-maintainer of the package; please add the epel-packagers-sig group through https://src.fedoraproject.org/rpms/python-nbformat/addgroup and grant it commit access, or collaborator access on epel* branches. I would also be happy to be a co-maintainer (FAS: jonathanspw). I can be the primary contact for EPEL (FAS: jonathanspw).
I've added you to the package. Thanks.