DeprecationWarning with traitlets when using %matplotlib widget

まず、提供された Python スニペットと %matplotlib widget バックエンドを使って警告を再現し、次に traceback に記載されている Canvas と Toolbar の初期化を追跡します。ipympl、ipywidgets、traitlets の記載されたバージョンで動作を比較します。完了条件は、例でこれらの DeprecationWarning メッセージが出力されなくなり、動作が適切な回帰チェックでカバーされていることです。

Describe the issue

It seems that the widget backend is doing something that is deprecated, and I thought I should make a note of it here.
Example:

import warnings
import matplotlib.pyplot as plt

%matplotlib widget
warnings.simplefilter("always", DeprecationWarning) 
plt.subplots()

I get the following warnings

/home/maij/.local/lib/python3.6/site-packages/ipywidgets/widgets/widget.py:412: DeprecationWarning: Passing unrecoginized arguments to super(Canvas).__init__().
__init__() missing 1 required positional argument: 'figure'
This is deprecated in traitlets 4.2.This error will be raised in a future release of traitlets.
  super(Widget, self).__init__(**kwargs)
/home/maij/.local/lib/python3.6/site-packages/ipywidgets/widgets/widget.py:412: DeprecationWarning: Passing unrecoginized 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)

Versions

 3.6.9 (default, Apr 18 2020, 01:56:04) 
[GCC 8.4.0]
ipympl version: 0.5.6
jupyter core     : 4.6.3
jupyter-notebook : 6.0.3
qtconsole        : 4.7.4
ipython          : 7.13.0
ipykernel        : 5.2.0
jupyter client   : 6.1.3
jupyter lab      : 2.1.4
nbconvert        : 6.0.0a0
ipywidgets       : 7.5.1
nbformat         : 5.0.4
traitlets        : 4.3.3
JupyterLab v2.1.4
Known labextensions:
   app dir: /usr/local/share/jupyter/lab
        @jupyter-widgets/jupyterlab-manager v2.0.0  enabled  OK
        @jupyter-widgets/jupyterlab-sidecar v0.5.0  enabled  OK
        jupyter-matplotlib v0.7.2  enabled  OK
        shared_kernel v0.1.0  enabled  OK