id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
391398977,MDU6SXNzdWUzOTEzOTg5Nzc=,2609,Cannot replace xr.ufuncs.angle with np.angle,630936,closed,0,,,2,2018-12-15T18:01:46Z,2018-12-20T18:01:38Z,2018-12-20T18:01:38Z,CONTRIBUTOR,,,,"Currently one gets a deprecation warning when using `xarray.ufuncs.angle` on a DataArray with xarray 0.11 and numpy 1.15, e.g.
```
PendingDeprecationWarning: xarray.ufuncs will be deprecated when xarray no longer supports versions of numpy older than v1.13. Instead, use numpy ufuncs directly.
    X_tr = xr.ufuncs.angle(X)
```

however, `np.angle` still casts the input to `ndarray`, apparently.

#### Code Sample, a copy-pastable example if possible


```python
>>> import xarray as xr
>>> import numpy as np
>>> X = xr.DataArray(np.ones(10))
>>> X
<xarray.DataArray (dim_0: 10)>
array([1., 1., 1., 1., 1., 1., 1., 1., 1., 1.])
Dimensions without coordinates: dim_0
>>> xr.ufuncs.angle(X)  # expected output
<xarray.DataArray (dim_0: 10)>
array([0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])
Dimensions without coordinates: dim_0
>>> np.angle(X)  # actual output
array([0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])
>>> np.__version__
'1.15.4'
>>> xr.__version__
'0.11.0'
>>> np.sin(X)  # other ufuns work
<xarray.DataArray (dim_0: 10)>
array([0.841471, 0.841471, 0.841471, 0.841471, 0.841471, 0.841471, 0.841471,
       0.841471, 0.841471, 0.841471])
Dimensions without coordinates: dim_0
```
#### Problem description

I have not investigated yet why happens in numpy, but opening this issue to indicate that currently I can't really do much about this DeprecationWarning sort of silencing it (unless I'm missing something).

#### Output of ``xr.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.18.8-gentoo
machine: x86_64
processor: Intel(R) Core(TM) i5-6200U CPU @ 2.30GHz
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

xarray: 0.11.0
pandas: 0.23.4
numpy: 1.15.4
scipy: 1.1.0
netCDF4: None
h5netcdf: 0.6.2
h5py: 2.8.0
Nio: None
zarr: None
cftime: None
PseudonetCDF: None
rasterio: None
iris: None
bottleneck: None
cyordereddict: None
dask: 1.0.0
distributed: None
matplotlib: 3.0.2
cartopy: None
seaborn: None
setuptools: 40.6.2
pip: 18.1
conda: None
pytest: 4.0.1
IPython: None
sphinx: None

</details>
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