For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ChrisRo0314:branches:systemsmanagement:katello/15.6/home:ChrisRo0314:branches:systemsmanagement:katello.repo
zypper refresh
zypper install python-decorator
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ChrisRo0314:branches:systemsmanagement:katello/15.5/home:ChrisRo0314:branches:systemsmanagement:katello.repo
zypper refresh
zypper install python-decorator