Christian, why PY_HASH_EXTERNAL is here? Do you plan use it any official build? I think that in custom build of Python whole files pyhash.c and pyhash.h can be replaced.

When you will get rid from PY_HASH_EXTERNAL, then you could get rid from PyHash_FuncDef, PyHash_Func, etc.

Why _Py_HashDouble() and _Py_HashPointer() are moved to pyhash.c? They are hash algorithm agnostic, and it is unlikely they will be redefined in custom build.

You not need the HAVE_ALIGNED_REQUIRED macros if use PY_UHASH_CPY (or something like for exact 64 bit) in siphash24. On platforms where aligned access is required you will use per-bytes copy, otherwise you will use fast 64-bit copy.