15.4 の場合は、 root で下記のコマンドを実行してください:
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
15.3 の場合は、 root で下記のコマンドを実行してください:
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