SHA512/224 and SHA512/256 are truncated versions of SHA512. Just like SHA384 they use the same algorithm but different initial values and a smaller digest. I took the start vectors and test values from libtomcrypt.

Like in my blake2 branch I have add tp_new to the types and removed the old factory methods. Now it is possible to instantiate the types.

The code is also in my github fork https://github.com/tiran/cpython/tree/feature/sha512truncated