MyBinder + ipympl: RuntimeError: JupyterLab failed to build

I'd like to play with Voila, MyBinder and ipympl. Unfortunately, I can't get ipympl to work with JupyterLab in MyBinder :slightly_frowning_face:

But actually, the issue can be reproduced with your MyBinder badge!

https://mybinder.org/v2/gh/matplotlib/ipympl/stable?urlpath=%2Flab%2Ftree%2Fexamples%2Fipympl.ipynb

Reproduce the failure using the linked MyBinder URL and the repository's MyBinder badge, opening examples/ipympl.ipynb in JupyterLab. Investigate why the JupyterLab build fails for ipympl; done means the example opens and ipympl works in that environment.

Written by the indexing model from the issue text.