Resize callback not working

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## Describe the issue
I am having an issue specifically with the Resize callback. The other callback options appear to work as expected. For example replacing `'resize_event'` with `'draw_event'` in the code block below gives no issues.

```python
import matplotlib.pyplot as plt
import numpy as np
from ipywidgets import Output
%matplotlib widget

output = Output()

@output.capture()
def key_event(event):
print('action')

fig, ax = plt.subplots()
ax.plot(np.random.rand(10))
fig.canvas.mpl_connect('resize_event', key_event)

display(output)
```

## Versions

```
3.6.8 (default, Jan 14 2019, 11:02:34)
[GCC 8.0.1 20180414 (experimental) [trunk revision 259383]]
ipympl version: 0.7.0
jupyter core : 4.5.0
jupyter-notebook : 6.0.1
qtconsole : not installed
ipython : 7.7.0
ipykernel : 5.1.2
jupyter client : 5.3.1
jupyter lab : 1.0.9
nbconvert : 5.6.0
ipywidgets : 7.6.3
nbformat : 4.4.0
traitlets : 4.3.2
Known nbextensions:
config dir: /usr/local/etc/jupyter/nbconfig
notebook section
ipyvolume/extension enabled
- Validating: OK
jupyter-datawidgets/extension enabled
- Validating: OK
jupyter-matplotlib/extension enabled
- Validating: OK
jupyter-threejs/extension enabled
- Validating: OK
jupyter-webrtc/extension enabled
- Validating: OK
jupyter-js-widgets/extension enabled
- Validating: OK
tree section
ipyparallel/main enabled
- Validating: OK
JupyterLab v1.0.9
Known labextensions:
app dir: /usr/local/share/jupyter/lab
jupyter-threejs v2.3.0 enabled OK
jupyterlab-datawidgets v7.0.0 enabled OK
```

Start by running the supplied Jupyter Notebook example with ipympl 0.7.0 and compare the `resize_event` callback with `draw_event`. Trace the `fig.canvas.mpl_connect` entry point through the notebook integration to identify why resizing does not reach the callback. Done means the example prints `action` when the figure is resized, with the existing callback behavior preserved.

Written by the indexing model from the issue text.