Unclear how to setup my own drag icon in a Gtk.Treeview

There is an open StackOverflow question about this with a minimal working example.

I want to use my own drag icon in a Gtk.Treeview. The official tutorial says that the drag-begin signal/event can be used for this. The API-Reference give more details about how to do it.

But in my example here it doesn't work. There is still the treeviews default drag icon used. It doesn't matter if I connect via connect() or connect_after().