Work around SWIG shadow instance creation bug in Python 3

SWIG's creation of shadow instances is broken in Python 3. It attempts
to call the 'object' constructor instead of using the tp_new function
that's present in the PyTypeObject struct. The allocation fails, which
crashes the Python interpreter (segfault from null pointer dereference)
when this object is used. I'm not the first person to report this to the
SWIG mailing list, but it doesn't seem to have received any developer
attention yet.

As far as I've seen, this is only used to wrap the librets exception
classes, so the end result of this is that any kind of RetsException
causes a segfault. It was very easy to reproduce this type of crash
by running any of the Python example code.

Until this is fixed in SWIG, patch the generated C++ code to create
Python objects correctly.