This has started failing because a transitive dependency unicode-ident updated their licenses to include Unicode-DFS-2016: https://github.com/dtolnay/unicode-ident/pull/9/files

This license seems very permissive so I think it should be ok for us to add to our list of permitted licenses, but we may want to check with legal first.

Also, while it isn't causing failures, cargo deny now produces a warning that there is no dependency matching the advisory criteria for RUSTSEC-2020-0159 so perhaps we can remove that from our list of advisories to ignore.