PyModule_AddObject has somewhat strange reference-counting behavior in
that it *conditionally* steals a reference.  In case of error it doesn't
change the reference to the passed object, but in case of success it
steals it.  This means that, as written, PyModule_AddIntConstant and
PyModuleAddStringConstant can leak created objects if PyModule_AddObject
fails.

As far as I can tell, the correct way to write those functions would be
(using PyModule_AddIntConstant as the example):

int 
PyModule_AddIntConstant(PyObject *m, const char *name, long value)
{
        PyObject *o = PyInt_FromLong(value);
	if (PyModule_AddObject(m, name, o) == 0)
                return 0;
        Py_XDECREF(o);
        return -1;
}

PyModule_AddObject was obviously intended to enable writing the "simple"
code (it even gracefully handles being passed NULL object to add) like
the one in PyModule_AddIntConstant, but I don't see a way to enable such
usage and avoid both leaks and an interface change.  Changing the
reference-counting behavior of PyModule_AddObject would be
backward-incompatible, but it might be a good idea to consider it for
Python 3.

If there is agreement that my analysis and the proposed fixes are
correct, I will produce a proper patch.