Update template for newest repo2docker

[pnasrat]

Context

I suspect the pins for the below are incorrect for latest repo2docker based images, this template should be updated - possibly as part of the image management work if it is going to continue to be the base for community images

  - jupyter_contrib_nbextensions==0.5.1
  # Required until https://github.com/jupyterhub/repo2docker/pull/1196 is merged
  - jupyterhub-singleuser>=3.0,<4.0

Proposal

No response

Updates and actions

No response

[weiji14]

Found this issue via the link to jupyterhub/repo2docker#1196 and was wondering what to do since that PR wasn't merged. Do we just keep using jupyterhub-singleuser on managed JupyterHubs? We're using jupyterhub-singleuser~=4.0.1 on CryoCloud, but I see a new version 5.1.0 already, so wondering if it's ok to just continue using jupyterhub-singleuser since it seems to be maintained and working ok?