Para 15.4 ejecute lo siguiente como root:
zypper addrepo https://download.opensuse.org/repositories/home:neotinker3:ROS:ROS1:noetic:external_dependencies:staging/15.4/home:neotinker3:ROS:ROS1:noetic:external_dependencies:staging.repo
zypper refresh
zypper install python-defusedxml
Para 15.3 ejecute lo siguiente como root:
zypper addrepo https://download.opensuse.org/repositories/home:neotinker3:ROS:ROS1:noetic:external_dependencies:staging/15.3/home:neotinker3:ROS:ROS1:noetic:external_dependencies:staging.repo
zypper refresh
zypper install python-defusedxml