Create 'combined' output dir if it doesn't exist

[cjduffett]

Overview

We typically don't check-in the combined output directory, only the local and managed directories, since combined policies can change dramatically between wonk builds. When running wonk in automated environments like Atlantis, the output dir may not exist at all or may be ephemeral.

I updated wonk to create the combined output dir if it doesn't already exist. I also updated all project dependencies, namely to relay the PyYAML constraint (so downstream dependencies may start using PyYAML 6.0+).

[briankzimmer]

👏

[kstrauser]

Looks good for me.