Incompatibility with xarray>=0.16

[TomAugspurger]

With siphon 0.8.0 and a call with use_xarray=True, I see an ImportError inside siphon:

In [56]: import siphon.cdmr.xarray_support
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-56-2438b4811c5c> in <module>
----> 1 import siphon.cdmr.xarray_support

~/miniconda3/envs/pangeo-forge-37/lib/python3.7/site-packages/siphon/cdmr/xarray_support.py in <module>
      7 from xarray.backends.common import AbstractDataStore, BackendArray
      8 from xarray.core import indexing
----> 9 from xarray.core.utils import FrozenOrderedDict
     10
     11 from . import Dataset

ImportError: cannot import name 'FrozenOrderedDict' from 'xarray.core.utils' (/Users/taugspurger/miniconda3/envs/pangeo-forge-37/lib/python3.7/site-packages/xarray/core/utils.py)

From http://xarray.pydata.org/en/stable/faq.html#what-parts-of-xarray-are-considered-public-api, xarray considers xarray.core to be private, so libraries probably shouldn't be importing from it.

Full example:

In [59]: from siphon.catalog import TDSCatalog
    ...:

In [60]: ds = TDSCatalog("https://geoport.usgs.esipfed.org/thredds/catalog/coawst_4/use/fmrc/catalog.xml").datasets[0].remote_access(use_xarray=True)
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-60-8d35c9e831d1> in <module>
----> 1 ds = TDSCatalog("https://geoport.usgs.esipfed.org/thredds/catalog/coawst_4/use/fmrc/catalog.xml").datasets[0].remote_access(use_xarray=True)

~/miniconda3/envs/pangeo-forge-37/lib/python3.7/site-packages/siphon/catalog.py in remote_access(self, service, use_xarray)
    620             raise ValueError(service + ' is not a valid service for remote_access')
    621
--> 622         return self.access_with_service(service, use_xarray)
    623
    624     def subset(self, service=None):

~/miniconda3/envs/pangeo-forge-37/lib/python3.7/site-packages/siphon/catalog.py in access_with_service(self, service, use_xarray)
    672         if service == 'CdmRemote':
    673             if use_xarray:
--> 674                 from .cdmr.xarray_support import CDMRemoteStore
    675                 try:
    676                     import xarray as xr

~/miniconda3/envs/pangeo-forge-37/lib/python3.7/site-packages/siphon/cdmr/xarray_support.py in <module>
      7 from xarray.backends.common import AbstractDataStore, BackendArray
      8 from xarray.core import indexing
----> 9 from xarray.core.utils import FrozenOrderedDict
     10
     11 from . import Dataset

ImportError: cannot import name 'FrozenOrderedDict' from 'xarray.core.utils' (/Users/taugspurger/miniconda3/envs/pangeo-forge-37/lib/python3.7/site-packages/xarray/core/utils.py)

[dopplershift]

Ugh, thanks for reporting. Definitely need to address and get a 0.9 out the door.

[RemiSahl]

Maybe something similar to what psyplot is doing could work ?
https://psyplot.readthedocs.io/en/latest/_modules/psyplot/gdal_store.html

try:
    from xarray.core.utils import FrozenOrderedDict
except ImportError:
    FrozenOrderedDict = dict

[dopplershift]

It's actually likely we do like MetPy, and bump our minimum supported xarray version and change the code to:

from xarray.core.utils import FrozenDict

[dopplershift]

Looks like this was actually already done in #291 , but just not released yet.