After trying it out, pulling SOCKET in the Python headers brings complications under Windows, because you need to include <winsock2.h> which in turn breaks compilation of _pickle.c (duplicate definition of "FLOAT", "INT", etc.).

An alternative approach would be to define only SIZEOF_SOCKET_T in the standard headers (we don't actually need access to the SOCKET type to compute it, as the current hard-coding shows), and leave SOCKET_T in socketmodule.h.