text: Title: Supporting JupyterHub singleuser 3.0.0 
  Body: I had a look at what's required for this to bump `jupyterhub-singleuser` to `3.0.0` (https://github.com/jupyterhub/binderhub/pull/1544#issuecomment-1278971454).
  
  JupyterHub 3.0.0 requires Python 3.7+
  
  ## Python 3.7, 3.8, 3.9, 3.10 environments
  This is easy, just update
  https://github.com/jupyterhub/repo2docker/blob/d7be04efb590a2a1227614ccaf9b504ad9333470/repo2docker/buildpacks/conda/environment.yml#L11
  and run [`freeze.py`](https://github.com/jupyterhub/repo2docker/blob/d7be04efb590a2a1227614ccaf9b504ad9333470/repo2docker/buildpacks/conda/freeze.py) to update Python 3.7, 3.8, 3.9 and 3.10 environments.
  
  ## Python 2.7 environment
  
  Also easy, since the kernel environment is decoupled from the notebook/JupyterLab environment.
  
  ## Python 3.5 environment
  Not automatically updated:
  https://github.com/jupyterhub/repo2docker/blob/d7be04efb590a2a1227614ccaf9b504ad9333470/repo2docker/buildpacks/conda/environment.py-3.5.yml#L1-L2
  Stuck on a very old version, too old for 3.0.0:
  https://github.com/jupyterhub/repo2docker/blob/d7be04efb590a2a1227614ccaf9b504ad9333470/repo2docker/buildpacks/conda/requirements.py-3.5.pip#L8
  
  ## Python 3.6 environment
  Not automatically updated:
  https://github.com/jupyterhub/repo2docker/blob/d7be04efb590a2a1227614ccaf9b504ad9333470/repo2docker/buildpacks/conda/environment.py-3.6.yml#L1-L2
  Stuck on an old version, too old for 3.0.0:
  https://github.com/jupyterhub/repo2docker/blob/d7be04efb590a2a1227614ccaf9b504ad9333470/repo2docker/buildpacks/conda/environment.py-3.6.lock#L131
  
  ## Options
  
  - Separate Python 3.5 and 3.6 environments from the notebook/JupyterLab environment, similar to Python 2.7
  - Separate all Python environments from the notebook/JupyterLab environment. If we're going to migrate 3.5 and 3.6 environments to the decoupled setup, it probably makes sense to do it for all Python versions to make things easier in the future, and also solves some potential package conflicts: https://github.com/jupyterhub/repo2docker/issues/741
  - Separate all environments for all languages from the notebook/JupyterLab environment. Architecturally I think this makes sense, but is more work: https://github.com/jupyterhub/repo2docker/issues/868