JupyterLab 3.0.0a10 /lab url doesn't work

[manics]

Bug description

Normally repo2docker defaults to launching Jupyter Notebook, but you can switch to JupyterLab by changing the URL path to /lab. This fails with JupyterLab 3.0.0a10 (the latest pre-release).
JupyterLab release schedule: jupyterlab/jupyterlab#8038

Expected behaviour

Going to /lab should take you to the JupyterLab interface

Actual behaviour

You get the Jupyter 404 page:

404 : Not Found

You are requesting a page that does not exist!

How to reproduce

Click on https://mybinder.org/v2/gist/manics/922d4e88fefc6d38fccea8cd2097bc28/master?urlpath=lab
Or run
repo2docker https://gist.github.com/manics/922d4e88fefc6d38fccea8cd2097bc28 and go to /url

Although the default will remain JupyterLab 2.0 for the immediate future I think we should ensure 3.0 works for anyone who wants to use it (#935)

[manics]

@blink1073 Do you know if this is something we should fix in the repo2docker launcher, or is it related to the lab integration with some other component?

[blink1073]

This is related to jupyterhub/jupyterhub#3128. The recommended way to use JuptyerLab 3.0 with JupyterHub is to use c.Spawner.cmd=["jupyter-labhub"]

[blink1073]

We fixed the ability to launch jupyterlab as a classic notebook extension in jupyterlab/jupyterlab#8956 (pending another beta release). We will eventually remove this capability once everyone has migrated to using jupyter_server.

[manics]

Thanks! I can confirm JupyterLab 3.0.0b3 works with repo2docker 👍

I've opened a separate issue for updating repo2docker to work with jupyter_server #956

[blink1073]

Great, thanks @manics!