add zarr-python v3 spec tests cases

[grlee77]

marked as a draft since this is using a branch corresponding to unmerged upstream PRs

Note that the environment file here installs from the branch corresponding to zarr-developers/zarr-python#898

[grlee77]

@joshmoore, could you take a look at the first three commits here in terms of the API for writing the files? In the first commit, I was trying the reuse the same high-level open function as for the v2 case, but for v3 it requires specifying a path ('astronaut' was used in that first commit). However, this makes an additional 'astronaut' subfolder within /meta/root and /data/root which is not present in the data generated from zarrita or xtensor-zarr.

The forms in commits 2 and 3 are equivalent, just using zarr.open_array or zarr.array directly.

The main issue is that in the current v3 implementation, one cannot create a Group without some for the group. i.e. the group cannot be at just /data/root/ but needs to be /data/root/<name>. In zarrita and xtensor-zarr there is a Hierarchy class that represents this top root level. The hierarchy just contains nodes corresponding to either groups or arrays. We can potentially also add this in zarr-python if it is helpful (specifically should zarr.open return a Hierarchy object in some cases where it currently returns a Group?)

[joshmoore]

Thanks, @grlee77. Yeah, I imagine anything you struggle with like this also points to issues that users will have, so we will need to engage with it seriously. Not that I have a solution at the moment, so the changes look within reason for the interim.

I've also hit on the desire for a hierarchy class a couple of times. I don't know if now is the time to do it or not (and perhaps just for V3). @constantinpape said something similar when comparing the API to Z5.

[joshmoore]

Could imagine that we even offer "default file ending" somewhere as a constant.