Exception not handled in widgetlock()

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## Describe the issue
I want to use a selector `matplotlib.widgets.RectangleSelector` , it is incompatible with zoom and pan
So I try to call widget lock to prevent user from using zoom in the same time.

minimum code:
```python
%matplotlib ipympl
import matplotlib.pyplot as plt
fig,ax = plt.subplots()
ax.plot([0,1],[0,1])
fig.canvas.widgetlock(ax) # get the widget lock
```
if you then hit the zoom or pan matplotlib button, which also try to get the lock, you get
```
ValueError Traceback (most recent call last)
File ~/spike/venvOW/lib/python3.9/site-packages/ipympl/backend_nbagg.py:279, in Canvas._handle_message(self, object, content, buffers)
276 self.manager.handle_json(content)
278 else:
--> 279 self.manager.handle_json(content)

...

File ~/spike/venvOW/lib/python3.9/site-packages/matplotlib/widgets.py:42, in LockDraw.__call__(self, o)
40 """Reserve the lock for *o*."""
41 if not self.available(o):
---> 42 raise ValueError('already locked')
43 self._owner = o

ValueError: already locked

```

Alternatively, I could use the right click for the selector, but I have not found a way to restrict zoom and pan to left click only.

## Versions

```
3.9.7 (default, Sep 16 2021, 13:09:58)
[GCC 7.5.0]
ipympl version: 0.8.8
Selected Jupyter core packages...
IPython : 8.0.1
ipykernel : 6.9.1
ipywidgets : 7.6.5
jupyter_client : 7.1.2
jupyter_core : 4.9.2
jupyter_server : 1.13.5
jupyterlab : not installed
nbclient : 0.5.11
nbconvert : 6.4.2
nbformat : 5.1.3
notebook : 6.4.8
qtconsole : 5.2.2
traitlets : 5.1.1
```

Reproduce the issue with the minimal Jupyter example, then inspect ipympl/backend_nbagg.py and matplotlib/widgets.py around the widget lock handling shown in the traceback. Confirm the expected behavior when zoom or pan attempts to acquire an occupied lock, and add coverage showing that the interaction no longer produces an uncaught ValueError.

Written by the indexing model from the issue text.