Rust is a transitive dependency for the build

[JoeJasinski]

Hey all,

Recently our builds started breaking because of the [build-system] in the pyproject.toml.

[build-system]
requires = ["jupyter_packaging~=0.7.9", "jupyterlab>=3.0.0rc13,==3.*", "setuptools>=40.8.0", "wheel"]
build-backend = "setuptools.build_meta"

The version of jupyterlab is not pinned, and it pulls in the latest version of jupyterlab during the build. New versions of jupyterlab depend on Rust (juptyerlab installs jupyter-server-ydoc which installs jupyter-ydoc which installs y-py which depends on Rust). Since the [build-system] does not respect whats installed in the current virtualenv, there's no way to specify an older version of jupyterhub or other deps, which don't depend on rust.

https://github.com/lckr/jupyterlab-variableInspector/blob/master/pyproject.toml#L2

I'm not sure of the best solution for this; is juptyerlab a requirement for the build? Could that be removed or pinned?