[gdb/python] Use py_{none,notimplemented} more often

Replace:
...
    {
      Py_INCREF (Py_NotImplemented);
      return Py_NotImplemented;
    }
...
with:
...
    return py_notimplemented ().release ();
...

Likewise for py_none.

Approved-By: Tom Tromey <tom@tromey.com>