Fix leak in nrncore_arg.

Because `PyObject_GetAttrString` returns a new reference [1], we're
responsible for calling DECREF on the `callable`.

[1]: https://docs.python.org/3/c-api/object.html#c.PyObject_GetAttrString

src/nrnpython/nrnpy_hoc.cpp

@@ -3335,7 +3335,7 @@ static char* nrncore_arg(double tstop) {
     if (modules) {
         PyObject* module = PyDict_GetItemString(modules, "neuron.coreneuron");
         if (module) {
-            PyObject* callable = PyObject_GetAttrString(module, "nrncore_arg");
+            auto callable = nb::steal(PyObject_GetAttrString(module, "nrncore_arg"));
             if (callable) {
                 PyObject* ts = Py_BuildValue("(d)", tstop);
                 if (ts) {