Fix a FutureWarning from xarray around timedelta decoding

xarray will in the future stop automatically decoding timedelta
variables. It has added a FutureWarning about this. This warning was
causing the latest dependency CI checks to fail.

This particular test is specifically about timedelta handling, so we
want to opt in to decoding timedelta values.

Author: mx-moth

tests/masking/test_utils.py

@@ -112,7 +112,7 @@ def test_find_fill_value_timedelta_with_missing_value(
         dataset_path, 'var',
         numpy.array([[0, 1], [2, missing_value]], dtype=numpy.float32))
 
-    with xarray.open_dataset(dataset_path) as dataset:
+    with xarray.open_dataset(dataset_path, decode_timedelta=True) as dataset:
         data_array = dataset['var']
         assert dataset['var'].dtype == numpy.dtype('timedelta64[ns]')
         fill_value = masking.find_fill_value(data_array)