> The calculation of long_hash assumes an unsigned temporary type to get
> correct results for the bit shifting and masking.

Yes, exactly.

> The calculation is
> done on the absolute value of the long and then the sign is applied. We
> either needed to (1) add an unsigned Py_hash_t type or (2) just use
> size_t and Py_ssize_t.

I like (2);  the use of Py_hash_t suggests to me that the type used for the hash is configurable independently of Py_ssize_t, which isn't true.

Also, with Py_hash_t it's no longer clear that printing a hash value (e.g., using PyErr_Format and friends) should use the '%zd' modifier.