Warnings with backend_nbagg

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## 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

Start by reproducing the warning with pytest and the ipympl.backend_nbagg.new_figure_manager_given_figure call shown in the issue. Trace the Toolbar initialization and its interaction with ipywidgets and traitlets. Done means the reproduction no longer emits the reported DeprecationWarning and the relevant pytest run passes.

Written by the indexing model from the issue text.