loading widget issues 2.2.9 with matched versions from readme

还没有人认领这个 Issue。

## Describe the issue

`loading` widget on `jupyter lab` (version 2.2.9) when plotting, e.g.

```python
%matplotlib widget
from matplotlib.pyplot import subplots
fig, ax = plt.subplots()
fig.show()
```

## Versions

```
JupyterLab v2.2.9
Known labextensions:
app dir: /home/casper/miniconda3/share/jupyter/lab
@axlair/jupyterlab_vim v0.12.4 enabled OK
@jupyter-widgets/jupyterlab-manager v2.0.0 enabled OK
jupyter-leaflet v0.13.3 enabled OK
jupyter-matplotlib v0.7.4 enabled OK
jupyterlab-plotly v4.14.1 enabled OK*
plotlywidget v4.14.1 enabled OK
```

`conda list ipympl` lists

```
(master) ✗ conda list | egrep "matplotlib|ipympl"
ipympl 0.5.8 pypi_0 pypi
matplotlib 3.3.1 pypi_0 pypi
```

which should work according to the readme

首先，在 JupyterLab v2.2.9 中使用 ipympl 0.5.8、matplotlib 3.3.1 和 jupyter-matplotlib 0.7.4 复现提供的绘图示例。将结果与 README 的兼容性指导进行比较；完成意味着已复现并理解加载小部件问题，并已确定经过验证的修复方案或明确的兼容性修正。

由索引模型根据 Issue 内容生成。