Fix errors in the test suite due to pytest warning changes (#1423)

* Fix errors in the test suite due to pytest warning changes

This is causing CI failures for some builds.

This is really a pytest bug (pytest-dev/pytest#2430),
but working around this on our end is easy enough (and cleans things up a
little).

* deprecated_call() -> warns(FutureWarning)

* another attempt

xarray/tests/__init__.py

@@ -128,8 +128,8 @@ def assertWarns(self, message):
         with warnings.catch_warnings(record=True) as w:
             warnings.filterwarnings('always', message)
             yield
-            assert len(w) > 0
-            assert any(message in str(wi.message) for wi in w)
+        assert len(w) > 0
+        assert any(message in str(wi.message) for wi in w)
 
     def assertVariableEqual(self, v1, v2):
         assert_equal(v1, v2)

xarray/tests/test_dataarray.py

@@ -2322,7 +2322,7 @@ def test_to_dataset_whole(self):
         self.assertDatasetIdentical(expected, actual)
 
         expected = Dataset({'bar': ('x', [1, 2])})
-        with self.assertWarns('order of the arguments'):
+        with pytest.warns(FutureWarning):
             actual = named.to_dataset('bar')
         self.assertDatasetIdentical(expected, actual)
 

xarray/tests/test_dataset.py

@@ -274,7 +274,7 @@ class Arbitrary(object):
             self.assertDatasetIdentical(expected, actual)
 
     def test_constructor_deprecated(self):
-        with self.assertWarns('deprecated'):
+        with pytest.warns(FutureWarning):
             DataArray([1, 2, 3], coords={'x': [0, 1, 2]})
 
     def test_constructor_auto_align(self):