use unicode strings

Partially Fixes #14695

DO NOT MERGE!

This is the last of 3 PRs to fix #14695
Other PRs merged and can be looked at here:
https://github.com/saltstack/salt/pull/14942
https://github.com/saltstack/salt/pull/15007

The highstate outputter is more complicated than the others and I'm not sure if I'm being overzealous with unicode 'u' prefix.

A review by @s0undt3ch and @basepi would be greatly appreciated.