I haven't yet studied the patch in detail but I have a few questions:

(1) are you sure it is safe not to INCREF the obj pointer in the
Py_buffer? in the cases handled by your patch we still hold a reference
to the original args tuple and dict before calling PyBuffer_Release(),
but some functions may want to store the Py_buffer object somewhere to
re-use it later. It would seem more logical for PyBuffer_FillInfo to
INCREF the obj, and for PyBuffer_Release to DECREF it and set it to NULL.

(2) is it necessary to call directly bf_getbuffer & the like or is there
a higher-level API to do it?

(3) didn't you forget to convert "PyArg_ParseTuple(args, "s#iO:sendto",
[...])" in sock_sendto?

(4) is it really necessary to do a special case with PyString_Check()
rather than rely on the string type's getbuffer method?