For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:python:Factory/15.6/devel:languages:python:Factory.repo
zypper refresh
zypper install python3-idle
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:python:Factory/15.5/devel:languages:python:Factory.repo
zypper refresh
zypper install python3-idle
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/devel:languages:python:Factory/15.4/devel:languages:python:Factory.repo
zypper refresh
zypper install python3-idle