> The Python wrappers EVPobject, EVPXOFobject and HMACobject are **not** safe to be transfered across subinterpreters.

No Python object must be shared between two interpreter and no object should be transfered form one interpreter to another. That's not specific to _hashlib, but a general guidelines for subinterpreters ;-)

Converting _hashlib should allow to have one _hashlib module instance per interpreter ;-) (Rather than sharing the same instance between all interpereters which caused so many issues!)