Pin zarr<3

[valeriupredoi]

Description

Zarr 3.0.x is out and they did a LOT of refactoring eg zarr.indexing has gone, with no replacement, see https://zarr.readthedocs.io/en/latest/user-guide/v3_migration.html

we stopped using Zarr in PyActive/pyfive so am just gonna pin to <3 in the other branches we have, we'll prob retire those branches anyway

Closes #237

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• All tests pass