>
> Have you had a chance to read http://www.python.org/dev/peps/pep-0452/ ,
too?

Overall pep 452 looks good but one thing popped out at me:

        >>> import hashlib
        >>> from Crypto.Hash import MD5
        >>> m = MD5.new()
        >>> isinstance(m, hashlib.CryptoHash)
        True

This suggests that there is an ABC hashlib.CryptoHash that non hashlib
implementations sound use but that is not spelled out anywhere as required
or recommended for third party hash implementations or why using it is a
good thing. I believe that is what you want to do with the ABC being
defined in this issue so I'd mention that explicitly in the pep.