Code review comment for ~racb/ubuntu-archive-tools:sru-release-freeze

Before landing, we need to decide where the freeze information should live, push my example freeze data repository there, and update the URL in sru-release.

This introduces a dependency on python3-yaml as that's the easiest way to load a set. I think that should be OK.

We're locking in the format of the data file a little bit here, but I don't think that should be a problem. In the future we might want freeze information bounded by dates, for example. But this should do for now, and if we do change the data format in the future, it shouldn't be a big deal just to ask SRU team members to update their checkout of ubuntu-archive-tools.