hashutil: drop all pre-3.6 blake2 workarounds
blake2s and blake2b have been provided by the stdlib hashlib since Python 3.6, and we declare 3.7 as minimum Python version supported.
Test Plan
toxed
Migrated from D6868 (view on Phabricator)
blake2s and blake2b have been provided by the stdlib hashlib since Python 3.6, and we declare 3.7 as minimum Python version supported.
toxed
Migrated from D6868 (view on Phabricator)