GTK4 accessible API has to be overridden
Description
It is required for attributes whose value is a list.
widget.update_property([Gtk.AccessibleProperty.LABEL], ['a label'])
works but
widget.update_relation([Gtk.AccessibleRelation.LABELLED_BY], [[labelWidget1, labelWidget2]]
is not possible.
Note that there is a bunch of gtk_accessible_..._init_value
to initialize the GValue with the type associated with an attribute.