Doesn't currently build on MyBinder

还没有人认领这个 Issue。

## Issue
**What is linked from launch badge in Readme doesn't currently build on MyBinder**

It seems it isn't properly seeing `package.json` for some reason..

## Steps to reproduce
Go to the main repo Readme page [here](https://github.com/matplotlib/ipympl#ipympl) and click the `launch binder` badge.

The end of a build example that fails is shown below (`...` on first line represents start of the log that is not shown):

```
...
Successfully installed ipympl-0.8.7 ipywidgets-7.7.5 jupyterlab-widgets-1.1.4 widgetsnbextension-3.6.4
Remote version of pip: 23.1.2
Local version of pip: 23.0.1
Was pip installed by pip? False
Removed build tracker: '/tmp/pip-build-tracker-zn9i3t0k'
/srv/conda/envs/notebook/lib/python3.10/site-packages/setuptools/config/setupcfg.py:516: SetuptoolsDeprecationWarning: The license_file parameter is deprecated, use license_files instead.
warnings.warn(msg, warning_class)
Installing /home/jovyan/ipympl/labextension -> jupyter-matplotlib
Building extension in /home/jovyan/ipympl
An error occurred.
FileNotFoundError: [Errno 2] No such file or directory: '/home/jovyan/ipympl/package.json'
See the log file for details: /tmp/jupyterlab-debug-22xaku6u.log
Removing intermediate container 9dfca785e89e
The command '/bin/sh -c ./.binder/postBuild' returned a non-zero code: 1
```

从仓库 README 中的 Launch Binder 徽章开始，检查失败日志中显示的 .binder/postBuild 命令。重现构建过程，并调查为什么在构建 JupyterLab 扩展时缺少 /home/jovyan/ipympl/package.json。当通过链接的徽章成功完成 Binder 构建时，即视为完成。

由索引模型根据 Issue 内容生成。