JupyterLab 3 support

[keller-mark]

Fixes #37 . JupyterLab 3 uses React ^17.0.1, while earlier versions depended on React ~16.9.0

This also adds a section to the docs which lists the jupyterlab and jupyter notebook versions which are compatible with the widget.

[ilan-gold]

@keller-mark I upgraded and didn't notice anything glaringly wrong. I did have an issue where going back and forth between different configs with scatterplots where they did not display after closing the first widget but I seriously doubt that is related. I tried downgrading (i.e reverting the environment) for this PR but got an error when trying to install/uninstall

 jupyter labextension install @jupyter-widgets/jupyterlab-manager

but I'm not sure this is relevant. Can you reproduce this issue either on an earlier release or now?

[ilan-gold]

Alternatively, could you provide directions for reverting the changes to this environment?

[keller-mark]

How to start a fresh environment to try to A) reproduce the original issue with JupyterLab 3 and B) test whether the changes in this PR fix it.

conda deactivate
conda env remove -n vitessce-jupyter-examples

A.

git checkout master
# in docs/notebooks
conda env create -f environment.yml
conda install -c conda-forge jupyterlab>=3.0.0
conda list

Check that the version of JupyterLab is greater than 3.0.0

pip install -e ../..
cd ../../js
npm install
npm run build
cd ../docs/notebooks
jupyter labextension install ../../js

# the error about the React versions should appear at this point

before trying part B, delete the environment again

conda deactivate
conda env remove -n vitessce-jupyter-examples

B.

git checkout keller-mark/jupyterlab-3
# in docs/notebooks
conda env create -f environment.yml
conda list

Check that the version of JupyterLab is greater than 3.0.0

pip install -e ../..
cd ../../js
npm install
npm run build
cd ../docs/notebooks
jupyter labextension install ../../js
jupyter lab

[ilan-gold]

Ok @keller-mark looks like the issue I saw doesn't have to do with this so this looks good to me. I'll file an issue here about it although not sure it's a -python problem

[ilan-gold]

Opened: #68