Matplotlib figures are not redrawn in the cell in which they are created

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## Describe the issue

Using an interactive figure's `canvas.draw()` method does not update the figure if it was created in the same cell as the draw call.

Figure created in the same cell:

Figure created in another cell:

## Reproduce

1. Open a new notebook in JupyterLab or Notebook
2. Insert the following code:
````
import matplotlib.pyplot as plt
import time

%matplotlib widget

fig = plt.figure()

for i in range(5):
x = list(range(i+2))
xx = [x**2 for x in x]
plt.clf()
plt.plot(x, xx)
fig.canvas.draw()

time.sleep(1)
````
3. Run the cell and observe that the plot does not show until the loop has exited.

Optional:
1. Move the for loop to a separate cell from the figure creation
2. Run cell and observe plot updates during the loop

This can be reproduced using the [scipy notebook docker stack](https://jupyter-docker-stacks.readthedocs.io/en/latest/using/selecting.html#jupyter-scipy-notebook).

## Versions

```
3.8.5 | packaged by conda-forge | (default, Jul 31 2020, 02:39:48)
[GCC 7.5.0]
ipympl version: 0.5.7
jupyter core : 4.6.3
jupyter-notebook : 6.0.3
qtconsole : not installed
ipython : 7.17.0
ipykernel : 5.3.4
jupyter client : 6.1.6
jupyter lab : 2.1.5
nbconvert : 5.6.1
ipywidgets : 7.5.1
nbformat : 5.0.7
traitlets : 4.3.3
Known nbextensions:
config dir: /opt/conda/etc/jupyter/nbconfig
notebook section
jupyter-matplotlib/extension enabled
- Validating: OK
jupyter-js-widgets/extension enabled
- Validating: OK
JupyterLab v2.1.5
Known labextensions:
app dir: /opt/conda/share/jupyter/lab
@bokeh/jupyter_bokeh v2.0.3 enabled OK
@jupyter-widgets/jupyterlab-manager v2.0.0 enabled OK
jupyter-matplotlib v0.7.3 enabled OK
```

Inizia eseguendo la riproduzione fornita nella stessa cella in JupyterLab o Notebook con il backend `%matplotlib widget`, quindi confrontala con la versione in celle separate. Il lavoro è completato quando `fig.canvas.draw()` aggiorna visibilmente la figura durante ogni iterazione del ciclo, invece che solo dopo l'uscita dal ciclo.

Scritto dal modello di indicizzazione a partire dal testo della issue.