JupyterLab 3.0+ Support

[traviscancode604]

Is your feature request related to a problem? Please describe. We are running JupyterLab 3.2.8 and cannot use nbcelltests because of an extension installation error related to jupyterlab_celltests.

Error message: The extension "jupyterlab_celltests" does not yet support the current version of JupyterLab.

Describe the solution you'd like Would like to be able to run pip install nbcelltests per the documentation and have the package work for JupyterLab 3.2.8.

Describe alternatives you've considered Alternative of downgrading JupyterLab version is not feasible.

Additional context Related to https://github.com/jpmorganchase/nbcelltests/issues/215

Conflicting Dependencies:
JupyterLab              Extension      Package
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/application
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/apputils
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/codeeditor
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/codemirror
>=5.2.8 <5.3.0          >=4.0.0 <5.0.0 @jupyterlab/coreutils
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/docmanager
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/filebrowser
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/launcher
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/mainmenu
>=3.2.8 <3.3.0          >=2.0.0 <3.0.0 @jupyterlab/notebook

[timkpaine]

We already support jupyterlab 3+, just might need to do a new release