Test with Zarr V3

[tomwhite]

Add a GH Actions workflow that runs Cubed tests using Zarr V3 storage. This will ensure that the upcoming work to update Zarr to support the V3 spec (see zarr-developers/zarr-python#1480) is working in Cubed.

I've done a preliminary test in https://github.com/tomwhite/cubed/tree/zarr-v3-testing, and the current V3 implementation works with Cubed. (Run with ZARR_V3_EXPERIMENTAL_API=1 pytest -vs)

The next step is to turn this into a workflow that runs against Zarr's V3 branch (when it is created).

cc @MSanKeys963 @jhamman

[tomwhite]

Zarr 3.0.0a0 was released last week. It's on PyPI, and can be installed by specifying --pre with pip install.

There's already a GH Actions workflow for this, which is currently failing (e.g. here's a recent run).

Some useful pre-requisites to getting Zarr V3 working are:

• Add normalize_shape since it is not in Zarr's public API #479
  • (Removes a dependency on an internal Zarr API)
• Implement virtual array indexing using ndindex #441
  • (Removes a dependency on BasicIndexer, another internal Zarr API)
• Ndindex indexing #481
  • (Removes more dependencies on internal Zarr indexing APIs)
• Pluggable backend storage #480
  • (Adds a mechanism to try out Zarr V3)

The following bugfixes are needed in Zarr:

• [v3] Array open with a Path fails in v3 zarr-developers/zarr-python#1975
• [v3] Support zero-dimensional arrays zarr-developers/zarr-python#1976
• [v3] Support zero-sized arrays zarr-developers/zarr-python#1977
• [v3] Mode 'r+' is not working zarr-developers/zarr-python#1978

[tomwhite]

Closing this as we have been running a CI job against Zarr v3 for a while now: https://github.com/cubed-dev/cubed/actions/workflows/zarr-v3-tests.yml.