So the problem is that readinto(view) might result in several references
to view? I don't think that can be solved on the memoryview side.

One could do:

   view = PyMemoryView_FromObject(b);
   // Lie about writability
   ((PyMemoryViewObject *)view)->view.readonly = 0;

   [...]

Then the view owns a reference to the bytes object. But that does not
solve the problem that writable memoryviews based on a readonly object
might be hanging around.