DeprecationWarning with traitlets when using %matplotlib widget

## 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:
```python
import warnings
import matplotlib.pyplot as plt

%matplotlib widget
warnings.simplefilter("always", DeprecationWarning)
plt.subplots()
```
I get the following warnings
```python-trace
/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
```

Start by reproducing the warning with the supplied Python snippet and the `%matplotlib widget` backend, then trace the Canvas and Toolbar initialization mentioned in the traceback. Compare behavior with the listed ipympl, ipywidgets, and traitlets versions; done means the example no longer emits these DeprecationWarning messages and the behavior is covered by an appropriate regression check.

Written by the indexing model from the issue text.