Para 15.6 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:crameleon:branches:devel:languages:python:backports/15.6/home:crameleon:branches:devel:languages:python:backports.repo
zypper refresh
zypper install python-modernize
Para 15.5 execute o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:crameleon:branches:devel:languages:python:backports/15.5/home:crameleon:branches:devel:languages:python:backports.repo
zypper refresh
zypper install python-modernize