After adding the py38-all-but-dask CI in #3919, it discovered a few backend issues:

• zarr:
  • open_zarr with chunks="auto" always tries to chunk, even if dask is not available (fixed in add a CI that tests xarray with all optional dependencies but dask #3919)
  • ZarrArrayWrapper.__getitem__ incorrectly passes the indexer's tuple attribute to _arrayize_vectorized_indexer (this only happens if dask is not available) (fixed in add a CI that tests xarray with all optional dependencies but dask #3919)
  • slice indexers with negative steps get transformed incorrectly if dask is not available Fix negative slicing of Zarr arrays #8674
• rasterio:
  • calling pickle.dumps on a Dataset object returned by open_rasterio fails because a non-serializable lock was used (if dask is installed, a serializable lock is used instead)