[PyGObject] Drag & Drop in a TreeView

[<c buhtz posteo jp>]

I am totally confused about this topic. I think I miss some fundamental
understanding on how PyGObject works here.

I want to drag & drop items of one TreeView inside itself. Not other
widgets or applications. I just want to drag item 2 and drop it between
item 4 and 5.

I found some resources about this topic

This states that DnD in a TreeView is different from the rest of
PyGObject concept.
https://python-gtk-3-tutorial.readthedocs.io/en/latest/drag_and_drop.html

Some incomplete, out-dated, not working or to complex examples
http://jcoppens.com/soft/howto/pygtk_dnd/index.en.php
https://stackoverflow.com/q/29673495/4865723
https://stackoverflow.com/q/13882076/4865723

I even don't know which one of the events I really need. There are to
much. I attached a minimal example with a two column (str, str)
treeview.

Can you give me a hint and a direction on which points I should focus
on?