zarr.create(dtype=str) has different dtype in 3 (<U) vs 2 (O)

[rabernat]

For Zarr 2.18.3

zarr.create(shape=10, dtype=str).dtype
# -> dtype('O')

For v3 branch @ 5a134bf

zarr.create(shape=10, dtype=str, zarr_version=2).dtype
# -> dtype('<U')

This is causing problems in Xarray for string compatibility.

Might have been broken by #2036

cc @TomAugspurger