silence a couple of warnings in tests

[max-sixty]

The only other warning is:

xarray/tests/test_dataset.py::TestDataset::test_convert_dataframe_with_many_types_and_multiindex
  /Users/maximilian/workspace/xarray/xarray/core/dataset.py:3146: FutureWarning: Converting timezone-aware DatetimeArray to timezone-naive ndarray with 'datetime64[ns]' dtype. In the future, this will return an ndarray with 'object' dtype where each element is a 'pandas.Timestamp' with the correct 'tz'.
  	To accept the future behavior, pass 'dtype=object'.
  	To keep the old behavior, pass 'dtype="datetime64[ns]"'.
    data = np.asarray(series).reshape(shape)
  /usr/local/lib/python3.7/site-packages/pandas/core/apply.py:286: FutureWarning: Converting timezone-aware DatetimeArray to timezone-naive ndarray with 'datetime64[ns]' dtype. In the future, this will return an ndarray with 'object' dtype where each element is a 'pandas.Timestamp' with the correct 'tz'.
  	To accept the future behavior, pass 'dtype=object'.
  	To keep the old behavior, pass 'dtype="datetime64[ns]"'.
    results[i] = self.f(v)

I'm not sure what we want to do here - potentially we should make a choice between:

• the worse behavior of object
• accepting the coercing to naive datetimes
• supporting pandas extension arrays

[dcherian]

@max-sixty See #2666

[max-sixty]

Great, thanks @dcherian

Lmk if we can merge this, then

[dcherian]

Ya sounds good.

Though this job is showing a bunch more: https://travis-ci.org/pydata/xarray/jobs/486320561. We can leave these for a future PR.

 /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/dask/array/blockwise.py:204: UserWarning: The da.atop function has moved to da.blockwise
    warnings.warn("The da.atop function has moved to da.blockwise")

xarray/tests/test_plot.py::TestPlot::test_2d_coords_line_plot
  /home/travis/build/pydata/xarray/xarray/tests/test_plot.py:213: PytestDeprecationWarning: The 'message' parameter is deprecated.
  (did you mean to use `match='some regex'` to check the exception message?)
  Please comment on https://github.com/pytest-dev/pytest/issues/3974 if you have concerns about removal of this parameter.
    with pytest.raises(ValueError, message='If x or y are 2D '):

xarray/tests/test_dataset.py::TestDataset::test_to_dict_with_numpy_attrs
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/numpy/lib/type_check.py:546: DeprecationWarning: np.asscalar(a) is deprecated since NumPy v1.16, use a.item() instead
    'a.item() instead', DeprecationWarning, stacklevel=1)
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/numpy/lib/type_check.py:546: DeprecationWarning: np.asscalar(a) is deprecated since NumPy v1.16, use a.item() instead
    'a.item() instead', DeprecationWarning, stacklevel=1)
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/numpy/lib/type_check.py:546: DeprecationWarning: np.asscalar(a) is deprecated since NumPy v1.16, use a.item() instead
    'a.item() instead', DeprecationWarning, stacklevel=1)

xarray/tests/test_dataarray.py::TestDataArray::test_to_and_from_cdms2_sgrid
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/numpy/ma/core.py:7705: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    if not np.all(xinf == filled(np.isinf(y), False)):
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/numpy/ma/core.py:7705: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    if not np.all(xinf == filled(np.isinf(y), False)):
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/numpy/ma/core.py:7705: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    if not np.all(xinf == filled(np.isinf(y), False)):
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/numpy/ma/core.py:7705: DeprecationWarning: elementwise comparison failed; this will raise an error in the future.
    if not np.all(xinf == filled(np.isinf(y), False)):
  /home/travis/miniconda/envs/test_env/lib/python3.6/site-packages/cdms2/axis.py:825: UserWarning: genutil module not present, was not able to determine if axis is level based on units
    "genutil module not present, was not able to determine if axis is level based on units")

3.5 is triggering this (https://travis-ci.org/pydata/xarray/jobs/486320560):

/home/travis/miniconda/envs/test_env/lib/python3.5/site-packages/numpy/ma/core.py:6447: MaskedArrayFutureWarning: In the future the default for ma.maximum.reduce will be axis=0, not the current None, to match np.maximum.reduce. Explicitly pass 0 or None to silence this warning.
  return self.reduce(a)

[max-sixty]

Thanks for listing those!

Am I understanding correctly that those are not from code that's in xarray, though?