temporarily pin dask (#4873)

Author: keewis

ci/requirements/environment-windows.yml

@@ -8,7 +8,7 @@ dependencies:
   # - cdms2  # Not available on Windows
   # - cfgrib  # Causes Python interpreter crash on Windows: https://github.com/pydata/xarray/pull/3340
   - cftime
-  - dask
+  - dask<2021.02.0
   - distributed
   - h5netcdf
   - h5py=2

ci/requirements/environment.yml

@@ -9,7 +9,7 @@ dependencies:
   - cdms2
   - cfgrib
   - cftime
-  - dask
+  - dask<2021.02.0
   - distributed
   - h5netcdf
   - h5py=2