Doesn't currently build on MyBinder

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 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

リポジトリの README にある Launch Binder バッジから始め、失敗ログに表示されている .binder/postBuild コマンドを調べてください。ビルドを再現し、JupyterLab 拡張機能のビルド時に /home/jovyan/ipympl/package.json が存在しない理由を調査してください。リンクされたバッジから Binder ビルドが正常に完了すれば完了です。