Remove the special casing of Py_None when converting the return value

Remove the special casing of Py_None when converting the return value
of the Python part of a callback function to C.  If it cannot be
converted, call PyErr_WriteUnraisable with the exception we got.
Before, arbitrary data has been passed to the calling C code in this
case.

(I'm not really sure the NEWS entry is understandable, but I cannot
find better words)