Fix Python reference leak to results struct

[lgarrison]

From #227, I think we have a Python reference leak to the results structure. Specifically, I think the following pattern results in a leak:

PyObject *ret = PyList_New(0);//create an empty list
...
return Py_BuildValue("(Od)", ret, c_api_time);

PyBuildValue adds a ref, but the PyList_New reference is never decremented. This PR decrements the ref count after building the tuple, before the return.

Once we verify the fix, I'll port it to the other modules.

[lgarrison]

This is confirmed: sys.getrefcount(extn_results) in DDrppi.py returns 3 instead of 2 without the fix.