Make psutil optional dependency of NBResuse

[krinsman]

Try to make psutil import optional. Still need to implement fronted fallback behavior if no psutil is found. (Or rather, need to better understand how JupyterLab statusbar looks for NBResuse output, to make sure that NBResuse "does nothing" in the way that JupyterLab expects when psutil is not installed.)

[jtpio]

Is this change mostly motivated by this issue: jupyterlab/jupyterlab#7818?

Should psutil still be listed as an extra require in the setup.py too?

https://github.com/yuvipanda/nbresuse/blob/783735980c8dc09907499fb4a1bd401b8d22adf8/setup.py#L12

[krinsman]

@jtpio You're right, it maybe shouldn't be listed in the setup.py anymore either, otherwise it would get installed automatically with NBResuse via PyPI.

On the other hand maybe it should since the "no behavior behavior" would only be desired once/if/when NBResuse was merged into jupyter_server or JupyterLab -- if someone is going out of their way to install NBResuse as a standalone project, then I imagine they probably want the full functionality, which does require PSutil.

Admittedly I hadn't given this that much though -- I haven't actually gotten around to testing this PR locally with the latest version of NBResuse to make sure it doesn't break anything.

[michaelaye]

could this merge be causing my recently appeared Errors on macOS? jupyterlab/jupyterlab#8478