Binder switching to Jupyter Server by default

A bit over a year ago, Binder switched to JupyterLab for the default UI, but left the default server as jupyter-notebook.

I believe it is now time to switch the server implementation to jupyter-server as well, as the use of the older jupyter-notebook server is beginning to cause problems. For the vast majority of users, this should have no visible effect. I believe it will also decrease the number of repos with problems, as compatibility issues with the old server are becoming more common than compatibility issues with the new one, but it does mean some repos that require the old server may start seeing some errors, so we want to give fair warning and solicit feedback. You should still be able to use the classic notebook UI on the new server via the nbclassic package, which is also installed by default.

BinderHub pull request switching the default server for discussion: switch default server to jupyter-server-based jupyter-lab by minrk · Pull Request #1635 · jupyterhub/binderhub · GitHub

Only images that contain JupyterLab will launch with the new server. If you use a Dockerfile in your repo and only install jupyter-notebook, it will still work with the old server.

Unfortunately, there’s no easy way for repos to express a preference in what server to launch, so this is a hard change that affects all users (there are hacky workarounds, however, for those who need a short-term fix).