Warnings with backend_nbagg

Issue

There is a warning being triggered when running pytest with the following instruction:

ipympl.backend_nbagg.new_figure_manager_given_figure(1, matplotlib.figure.Figure()))

The warning is the following:

 /Users/luisaleixo/.pyenv/versions/3.9.6/envs/cara/lib/python3.9/site-packages/ipywidgets/widgets/widget.py:477: DeprecationWarning: Passing unrecognized arguments to super(Toolbar).__init__().
  __init__() missing 1 required positional argument: 'canvas'
  This is deprecated in traitlets 4.2.This error will be raised in a future release of traitlets.
    super(Widget, self).__init__(**kwargs)

It is not possible to downgrade the triatlets version for this project.

Versions

Name: ipympl
Version: 0.9.1

Name: pytest
Version: 6.2.5

まず、issue に示されている pytest と ipympl.backend_nbagg.new_figure_manager_given_figure の呼び出しを使って警告を再現します。Toolbar の初期化と、ip ywidgets および traitlets との相互作用を追跡します。再現時に報告された DeprecationWarning が出力されなくなり、関連する pytest の実行が成功すれば完了です。