[BUG] new versions of xarray raise a future warning

[rcaneill]

/home/docs/checkouts/readthedocs.org/user_builds/xnemogcm/envs/100/lib/python3.10/site-packages/xgcm/grid_ufunc.py:832: FutureWarning: The return type of `Dataset.dims` will be changed to return a set of dimension names in future, in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, please use `Dataset.sizes`.
  out_dim: grid._ds.dims[out_dim] for arg in out_core_dims for out_dim in arg

[rcaneill]

/home/docs/checkouts/readthedocs.org/user_builds/xnemogcm/envs/100/lib/python3.10/site-packages/xnemogcm/nemo.py:78: FutureWarning: The return type of `Dataset.dims` will be changed to return a set of dimension names in future, in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, please use `Dataset.sizes`.
  z_nme = [i for i in ds.dims.keys() if "depth" in i][0]
/home/docs/checkouts/readthedocs.org/user_builds/xnemogcm/envs/100/lib/python3.10/site-packages/xnemogcm/nemo.py:84: FutureWarning: The return type of `Dataset.dims` will be changed to return a set of dimension names in future, in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, please use `Dataset.sizes`.
  x_nme = [i for i in ds.dims.keys() if "x_grid" in i or i == "x"]
/home/docs/checkouts/readthedocs.org/user_builds/xnemogcm/envs/100/lib/python3.10/site-packages/xnemogcm/nemo.py:86: FutureWarning: The return type of `Dataset.dims` will be changed to return a set of dimension names in future, in order to be more consistent with `DataArray.dims`. To access a mapping from dimension names to lengths, please use `Dataset.sizes`.
  y_nme = [i for i in ds.dims.keys() if "y_grid" in i or i == "y"]