#12 Update PyObject Buffer Protocol

[Robert Arkiletian]

This patch is against r536. For reading image buffers PyObject_asReadBuffer is deprecated in python 3
see https://docs.python.org/3/c-api/objbuffer.html
warning: ‘int PyObject_AsReadBuffer(PyObject, const void, Py_ssize_t)’ is deprecated [-Wdeprecated-declarations]
This patch uses the newer PyObject_GetBuffer
see https://docs.python.org/3/c-api/buffer.html

I have only done very minimal testing of this patch. The pertinent programs in test dir seem to be ok but this patch needs to be verified/tested.

[Andreas Held]

Applied in r540.