sysutils/py-salt: python3.8 is not supported yet

https://docs.saltstack.com/en/2019.2/topics/installation/index.html#dependencies
https://docs.saltstack.com/en/3000/topics/releases/3000.html#python-3-8-not-supported
https://github.com/saltstack/salt/issues/55310

PR: 243610
Reported by: Tommy P <tommyhp2@gmail.com>
Approved by: Christer Edwards <christer.edwards@gmail.com> (maintainer)