Open multiple groups (e.g. as DataTree) with zarr-python v3

[TomNicholas]

What is your issue?

In #9552 we punted (#9552 (comment)) on making this work with zarr-python v3 because there were some upstream bugs with zarr-python v3 not correctly handling keys with slashes in them (zarr-developers/zarr-python#2357). Once that's handled properly we can ensure that the zarr backend works for open_groups and open_datatree.

cc @jhamman

[jhamman]

The upstream issues are all sorted now so we should be good to go. Things may just work if we stop skipping the Datatree zarr tests for v3.