Modernize DECREF (part 1). (#3203)

This commit refactors a part of the NRN Python bindings to use `nanobind`
objects instead of `Py_DECREF`. The purpose is to simplify the DECREFing logic
on error paths; and reduce the risk of leaking when exceptions are thrown.

As part of the refactoring, if needed, the scope of certain variables might be
reduced or a given a new name. Additionally, NULL pointers are replaced with
`nullptr`.

This commit doesn't intentionally change reference counts.

src/nrnpython/nrnpy_nrn.cpp

@@ -404,13 +404,12 @@ static int NPySecObj_init(NPySecObj* self, PyObject* args, PyObject* kwds) {
             if (cell) {
                 // include cellname in name so nrnpy_pysecname2sec_remove can determine
 
-                cell = PyObject_Str(cell);
-                if (cell == NULL) {
+                auto cell_str = nb::steal(PyObject_Str(cell));
+                if (!cell_str) {
                     Py_XDECREF(self->cell_weakref_);
                     return -1;
                 }
-                Py2NRNString str(cell);
-                Py_DECREF(cell);
+                Py2NRNString str(cell_str.ptr());
                 if (str.err()) {
                     str.set_pyerr(PyExc_TypeError, "cell name contains non ascii character");
                     return -1;
@@ -453,33 +452,29 @@ static int NPyAllSegOfSecIter_init_safe(NPyAllSegOfSecIter* self, PyObject* args
 }
 
 PyObject* NPySecObj_new(PyTypeObject* type, PyObject* args, PyObject* kwds) {
-    NPySecObj* self;
-    self = (NPySecObj*) type->tp_alloc(type, 0);
-    // printf("NPySecObj_new %p\n", self);
-    if (self != NULL) {
-        if (NPySecObj_init(self, args, kwds) != 0) {
-            Py_DECREF(self);
-            return NULL;
+    auto self = nb::steal(type->tp_alloc(type, 0));
+    // printf("NPySecObj_new %p\n", self.ptr());
+    if (self) {
+        if (NPySecObj_init((NPySecObj*) self.ptr(), args, kwds) != 0) {
+            return nullptr;
         }
     }
-    return (PyObject*) self;
+    return self.release().ptr();
 }
 
 PyObject* NPySecObj_new_safe(PyTypeObject* type, PyObject* args, PyObject* kwds) {
     return nrn::convert_cxx_exceptions(NPySecObj_new, type, args, kwds);
 }
 
 PyObject* NPyAllSegOfSecIter_new(PyTypeObject* type, PyObject* args, PyObject* kwds) {
-    NPyAllSegOfSecIter* self;
-    self = (NPyAllSegOfSecIter*) type->tp_alloc(type, 0);
-    // printf("NPyAllSegOfSecIter_new %p\n", self);
-    if (self != NULL) {
-        if (NPyAllSegOfSecIter_init(self, args, kwds) != 0) {
-            Py_DECREF(self);
-            return NULL;
+    auto self = nb::steal(type->tp_alloc(type, 0));
+    // printf("NPyAllSegOfSecIter_new %p\n", self.ptr());
+    if (self) {
+        if (NPyAllSegOfSecIter_init((NPyAllSegOfSecIter*) self.ptr(), args, kwds) != 0) {
+            return nullptr;
         }
     }
-    return (PyObject*) self;
+    return self.release().ptr();
 }
 
 PyObject* NPyAllSegOfSecIter_new_safe(PyTypeObject* type, PyObject* args, PyObject* kwds) {
@@ -1860,17 +1855,16 @@ static PyObject* mech_of_segment_iter(NPySegObj* self) {
     CHECK_SEC_INVALID(sec)
     Node* nd = node_exact(sec, self->x_);
     Prop* p = mech_of_segment_prop(nd->prop);
-    NPyMechOfSegIter* mi = PyObject_New(NPyMechOfSegIter, pmech_of_seg_iter_generic_type);
+    auto mi = nb::steal((PyObject*) PyObject_New(NPyMechOfSegIter, pmech_of_seg_iter_generic_type));
     if (!mi) {
-        return NULL;
+        return nullptr;
     }
-    NPyMechObj* m = new_pymechobj(self, p);
+    auto m = nb::steal((PyObject*) new_pymechobj(self, p));
     if (!m) {
-        Py_XDECREF(mi);
-        return NULL;
+        return nullptr;
     }
-    mi->pymech_ = m;
-    return (PyObject*) mi;
+    ((NPyMechOfSegIter*) mi.ptr())->pymech_ = (NPyMechObj*) m.release().ptr();
+    return mi.release().ptr();
 }
 
 static PyObject* mech_of_segment_iter_safe(NPySegObj* self) {