Fix mixins error with new yaml dumper
Fix mixins error with new yaml dumper

In #42369 we introduced a new indented yaml dumper. However, the base
classes did not match which broke the docs build. This changes
ensures that the new `IndentMixin` inherits from the SafeDumper
and then overrides `increase_indent`.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt