Fix test suite with pandas 0.23 (#2136)

* Fix test suite with pandas 0.23

* Disable -OO check

Author: shoyer

.travis.yml

@@ -101,7 +101,9 @@ install:
   - python xarray/util/print_versions.py
 
 script:
-  - python -OO -c "import xarray"
+  # TODO: restore this check once the upstream pandas issue is fixed:
+  # https://github.com/pandas-dev/pandas/issues/21071 
+  # - python -OO -c "import xarray"
   - if [[ "$CONDA_ENV" == "docs" ]]; then
       conda install -c conda-forge sphinx sphinx_rtd_theme sphinx-gallery numpydoc;
       sphinx-build -n -j auto -b html -d _build/doctrees doc _build/html;

xarray/tests/test_dataset.py

@@ -1848,7 +1848,9 @@ def test_drop_index_labels(self):
         expected = data.isel(x=slice(0, 0))
         assert_identical(expected, actual)
 
-        with pytest.raises(ValueError):
+        # This exception raised by pandas changed from ValueError -> KeyError
+        # in pandas 0.23.
+        with pytest.raises((ValueError, KeyError)):
             # not contained in axis
             data.drop(['c'], dim='x')