For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/systemsmanagement:Uyuni:AI:Deps:devel:languages:python:backports/16.0/systemsmanagement:Uyuni:AI:Deps:devel:languages:python:backports.repo
zypper refresh
zypper install python-snowballstemmer
For 15.7 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/systemsmanagement:Uyuni:AI:Deps:devel:languages:python:backports/15.7/systemsmanagement:Uyuni:AI:Deps:devel:languages:python:backports.repo
zypper refresh
zypper install python-snowballstemmer