Better method for managing python black package

We are able to call `black` by shelling out to it on the system path, and we are able to cache its results by enforcing a version check on the binary. However, if it isn't on the path, or the version is wrong, we just show a helpful error message and let the user figure it out from there:

https://github.com/diffplug/spotless/blob/608e128381c89260f8a38fba985415c1f962ec7b/lib/src/main/java/com/diffplug/spotless/python/BlackStep.java#L65-L66

It would be nice if we could handle this better, or at least have better instructions. There are a few python plugins in the gradle ecosystem, I'll reach out to them to see if they are interested.