[BootstrapInfo](https://github.com/andsens/bootstrap-vz/blob/1781eef692efd730f8b056502cf307b4cb1e5be9/bootstrapvz/base/bootstrapinfo.py#L37) [normalizes the release name to a codename](https://github.com/andsens/bootstrap-vz/blob/1781eef692efd730f8b056502cf307b4cb1e5be9/bootstrapvz/base/release-codenames.json).
You can just use `info.release_codename`, which will always be 'sid' and never 'unstable'