Review packaging dependency

[jeromekelleher]

#322 fixed a problem where we have an undeclared dependency on the packaging package. While we can certainly have a dependency on this, it seems like overkill to me as we're just looking at Zarr. Can we not just look at the Zarr version strings ourselves?

[tomwhite]

It's very annoying that such a fundamental thing (parsing package versions) is not a part of Python. (I thought it was!) It's also very hard to search for this package to read about it!

Anyway, I'm fine either way on this one - having another dependency (very lightweight) or parsing the string ourselves.