Suppress xarray warning in test suite (#179)

- Importing `xarray.tests` sets `warn_for_unclosed_files` to True
- Must import utilities from `xarray.tests` `__init__.py` to avoid `warn_for_unclosed_files` to be reset back to True

tests/__init__.py

@@ -1,4 +1,5 @@
 """Unit test package for xcdat."""
 from xarray.core.options import set_options
+from xarray.tests import requires_dask  # noqa: F401
 
 set_options(warn_for_unclosed_files=False)

tests/test_spatial_avg.py

@@ -1,8 +1,8 @@
 import numpy as np
 import pytest
 import xarray as xr
-from xarray.tests import requires_dask
 
+from tests import requires_dask
 from tests.fixtures import generate_dataset
 from xcdat.spatial_avg import SpatialAverageAccessor