Test against ipywidgets master

还没有人认领这个 Issue。

## Describe the issue
Now that ipywidgets master is being updated again ipympl should run a weekly workflow checking that nothing breaks. This could either be in the workflow that runs against dev matplotlib or in a separate one.

This might be as simple as:
```
pushd $(mktemp -d)
git clone https://github.com/jupyter-widgets/ipywidgets.git --single-branch --depth 1
cd ipywidgets
bash dev-install.sh
popd
```

首先定位针对开发版 matplotlib 进行测试的 workflow，并确定 ipywidgets master 应该归入其中还是放在单独的 workflow 中。检查提议的 clone 和 dev-install 流程，然后验证该 workflow 是否每周运行，并能检测出 ipympl 针对当前 ipywidgets master 的故障。

由索引模型根据 Issue 内容生成。