Fix shadow instance creation failure in Python 3

I managed to trace a very nasty Python interpreter segfault to an
allocation failure here. Adding this after the tp_new call:
if (PyErr_Occurred()) {
    PyErr_Print();
}

results in output of 'TypeError: object() takes no parameters', followed
by a segfault that takes down the Python interpeter.

The 'object' constructor doesn't seem to be suitable for instantiating
SWIG shadow instances in this way, so simply use the constructor
function in the PyTypeObject 'tp_new' slot of data->newargs.

The 'if (inst)' check after this doesn't hurt in as much as it prevented
a segfault immediately after this failed allocation, but it doesn't help
much since the null pointer dereference will probably happen sooner or
later anyway.