For resuming cluster management from disk (e.g. getting a Cluster API handle on a cluster started from another process). This is going to mean serializing Launchers with e.g. PIDs, class names, etc. which in turn means that signalling / etc. must use os.kill or psutil, and not the subprocess.Popen object directly.

We do this already in JupyterHub's LocalProcessSpawner, but critically don't need to support Windows over there.