Helm command is now a mandatory dependency

[jacobtomlinson]

Unless I'm wrong, this implicitly makes helm a new mandatory dependency of dask-kubernetes, if that's the case, I think the doc should be updated to mention that.

Originally posted by @Timost in #255 (comment)

[jacobtomlinson]

Thanks for raising the flag here @Timost.

It should be an optional dependency for folks who want to use the HelmCluster class. However it seems I've introduced a bug which makes it mandatory.

dask-kubernetes/dask_kubernetes/helm.py

Lines 14 to 19 in 89a3760

	if shutil.which("helm") is None:
	raise ImportError(
	"Missing dependency helm. "
	"Please install helm following the instructions for your OS. "
	"https://helm.sh/docs/intro/install/"
	)

This probably shouldn't be module scoped. It should only raise for folks using HelmCluster.

[Timost]

No problem. I got caught by surprise when I updated my dask-kubernetes dependency yesterday, I also think it should be optional, I tend to consider Helm a non trivial dependency especially before helm v3 (because of tiller)

[jacobtomlinson]

Absolutely, the hard dependency was an accident. It was always meant to be optional. The problem came from importing HelmCluster at the top level for convenience.

Yes I am glad helm 3 made things much easier.