Quick update: change which HKDF function is used by HMAC-SHA256 verification machinery in PyRosettaCluster (#655)
This quick patch has virtually no effect on security or runtime
functionality, it just updates which extract-and-expand hash-based
message authentication code (HMAC)-based key derivation function (HKDF)
is used for pseudorandom key generation in task dictionary HMAC-SHA256
verification, reflecting original intention.