Fix reference counting leak in nrnpy_restore_savestate_.

The facts:
  1. `PyByteArray_FromStringAndSize` returns a "new reference" [1].
  2. `PyObject_CallObject` returns a "new reference" [2].
  3. `PyTuple_SetItem` steals `py_data` [3].

Let's analyse `py_data` first:
  * After `PyByteArray_FromStringAndSize` it's at `+1` (meaning 1
    unaccounted INCREF).
  * After `Py_INCREF(py_data)` it's at `+2`.
  * By calling `PyTuple_SetItem` we know 1 DECREF will happen, because
    it steals `py_data`. The reference count of `py_data` is at `+1`.

Therefore, there's one INCREF for which we can't pair up with a DECREF.

Let's move on to `result`, since it's a "new reference" we're
responsible for calling `DECREF`.

[1]: https://docs.python.org/3/c-api/bytearray.html#c.PyByteArray_FromStringAndSize
[2]: https://docs.python.org/3/c-api/call.html#c.PyObject_CallObject
[3]: https://docs.python.org/3/c-api/tuple.html#c.PyTuple_SetItem

src/nrnpython/nrnpy_hoc.cpp

@@ -2638,16 +2638,15 @@ static void nrnpy_store_savestate_(char** save_data, uint64_t* save_data_size) {
 static void nrnpy_restore_savestate_(int64_t size, char* data) {
     if (restore_savestate_) {
         PyObject* args = PyTuple_New(1);
-        PyObject* py_data = PyByteArray_FromStringAndSize(data, size);
-        Py_INCREF(py_data);
-        if (py_data == NULL) {
+        auto py_data = nb::steal(PyByteArray_FromStringAndSize(data, size));
+        if (!py_data) {
             hoc_execerror("SaveState:", "Data restore failure.");
         }
         // note: PyTuple_SetItem steals a ref to py_data
-        PyTuple_SetItem(args, 0, py_data);
-        PyObject* result = PyObject_CallObject(restore_savestate_, args);
+        PyTuple_SetItem(args, 0, py_data.release().ptr());
+        auto result = nb::steal(PyObject_CallObject(restore_savestate_, args));
         Py_DECREF(args);
-        if (result == NULL) {
+        if (!result) {
             hoc_execerror("SaveState:", "Data restore failure.");
         }
     } else {