feat(pkg/lint): flag packages that depend on unversioned python packages

[sil2100]

Depending on unversioned py3 module packages can lead to various hard-to-debug issues. We can and should be more explicit on which python version we are supporting, making sure we pull in the right module version at any time.

[sil2100]

I made this SeverityError as I do not know if SeverityWarning provides any feedback to the developer on PRs. Was thinking of an exception mechanism, but this would require a bit more work as we do not seem to save comment fields from the yaml.Node information into the parsed config structures. I don't know if we even need these.

[justinvreeland]

Looks fine to me. We'll want to audit the packages before making this enforcing or we'll cause breakage.