Reference to removed internal xarray module `xarray.core.pycompat`.

[aulemahal]

cf-xarray/cf_xarray/utils.py

Line 21 in 2b33ce5

from xarray.core.pycompat import is_duck_dask_array

This module was removed in xarray 2024.02.0 (I can't find the exact commit).

We could copy the removed code to cf-xarray ?