OutputWidget: use public API to get parent header
OutputWidget: use public API to get parent header

newly public in 6.0, Kernel._parent_header is private and deprecated in 6.0

ipython/ipywidgets,jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets