I don't know the policy of package inclusion in the repositories. The plpython3 package exists in the Fedora repository:
http://mirror.globo.com/fedora/linux/updates/22/x86_64/p/
But it does not exist in the Postgresql repository:http://mirror.globo.com/fedora/linux/updates/22/x86_64/p/
https://download.postgresql.org/pub/repos/yum/9.5/fedora/fedora-22-x86_64/
Or as source:
https://download.postgresql.org/pub/repos/yum/srpms/9.5/fedora/fedora-22-x86_64/