doctests fail with the new version of xarray

We build with xarray 0.18.0 and doctests pass there, but when tested with xarray 0.19.0 which is the latest in the defaults channel, the doctests fail. Since I cannot create a doctest that works with both of them, I am suggesting to pin xarray till the next minor version (instead of next major version).

See: https://gitlab.idiap.ch/bob/bob.pipelines/-/jobs/243826