An update a year later: I have a proof-of-concept branch that adds BLAKE3 support to hashlib: https://github.com/oconnor663/cpython/tree/blake3. That branch is API compatible with the current master branch of https://github.com/oconnor663/blake3-py. Both that module and the upstream BLAKE3 repo are ready to be tagged 1.0, just waiting to see whether any integrations like this one end up requesting changes.

Would anyone be interested in moving ahead with this? One of the open questions would be whether CPython would vendor the BLAKE3 optimized assembly files, or whether we'd prefer to stick to C intrinsics.