text: Title: Fails to install with miniconda on Windows
  Body: ### Operating system
  Windows 10
  
  ### `nbgrader --version`
  n/a since the installation failed
  
  ### `jupyterhub --version` (if used with JupyterHub)
  ```
  (base) C:\Users\Stefnotch>jupyterhub --version
  'jupyterhub' is not recognized as an internal or external command,
  operable program or batch file.
  ```
  
  ### `jupyter notebook --version` 
  ```
  (base) C:\Users\Stefnotch>jupyter notebook --version
  6.4.0
  ```
  
  ### Expected behavior
  I expected nbgrader to get installed
  
  
  ### Actual behavior
  
  ```
  (base) C:\Users\Stefnotch>conda install -c conda-forge nbgrader
  Collecting package metadata (current_repodata.json): done
  Solving environment: failed with initial frozen solve. Retrying with flexible solve.
  Solving environment: failed with repodata from current_repodata.json, will retry with next repodata source.
  Collecting package metadata (repodata.json): done
  Solving environment: failed with initial frozen solve. Retrying with flexible solve.
  Solving environment: /
  Found conflicts! Looking for incompatible packages.
  This can take several minutes.  Press CTRL-C to abort.
  Examining nbgrader:  80%|███████████████████████████████████████████████████▏            | 4/5 [00:00<00:00, 14.02it/s]/failed                                                                                                               /
  
  UnsatisfiableError: The following specifications were found
  to be incompatible with the existing python installation in your environment:
  
  Specifications:
  
    - nbgrader -> python[version='>=2.7,<2.8.0a0|>=3.6,<3.7.0a0|>=3.7,<3.8.0a0|>=3.8,<3.9.0a0']
  
  Your python: python=3.9
  
  If python is on the left-most side of the chain, that's the version you've asked for.
  When python appears to the right, that indicates that the thing on the left is somehow
  not available for the python version you are constrained to. Note that conda will not
  change your python version to a different minor version unless you explicitly specify
  that.
  ```
  
  ### Steps to reproduce the behavior
  1. Start up Windows 10
  2. Install miniconda
  3. `conda install -c conda-forge jupyterlab`
  4. `conda install jupyter`
  5. `conda install -c conda-forge nbgrader`
  
  