zarr-python (v3 branch) implementation

[joshmoore]

Integrate @Carreau ’s zarr-python implementation (https://github.com/Carreau/zarr-python/tree/spec-v3) into the zarr_implementations repository, generating both .zr and .zr3 files, again for all currently implemented codecs. This work may need to remain open as a PR if tests are failing as work on the spec-v3 branch continues. The goal is to have all V2 and V3 files readable by this branch.

[grlee77]

I did rebase that branch, fixing merge conflicts and then proposed some small changes in Carreau/zarr-python#1.

My updates were mainly modifying the list_dirs method of BaseV3Store to match the description in the spec and adding the metadata_key_suffix entry to zarr.json.

Using zarr.open does not yet seem to work for opening zr3 files. @Carreau, is there another way of opening these currently or should I hold off a bit on trying that?

Also, is there a plan to have hierarchy methods matching the API from zarrita and xtensor-zarr? e.g. create_hierarchy/get_hierarchy functions? (Such hierarchy methods are not described in the v3 spec so I wasn't sure)