Avoid running test_open_mfdataset_list_attr without dask (#3780)

Fixes GH-3777.

Author: ArchangeGabriel

doc/whats-new.rst

@@ -37,6 +37,9 @@ Internal Changes
 - Removed the internal ``import_seaborn`` function which handled the deprecation of
   the ``seaborn.apionly`` entry point (:issue:`3747`).
   By `Mathias Hauser <https://github.com/mathause>`_.
+- Changed test_open_mfdataset_list_attr to only run with dask installed
+  (:issue:`3777`, :pull:`3780`).
+  By `Bruno Pagani <https://github.com/ArchangeGabriel>`_.
 
 .. _whats-new.0.15.0:
 

xarray/tests/test_backends.py

@@ -2540,6 +2540,7 @@ def test_open_mfdataset_manyfiles(
 
 
 @requires_netCDF4
+@requires_dask
 def test_open_mfdataset_list_attr():
     """
     Case when an attribute of type list differs across the multiple files