Bug#871208: python-xarray tests failures

[Rebecca N. Palmer]

Control: tags -1 upstream fixed-upstream

This appears to be 2 separate bugs, both triggered by using pandas 0.20 
and fixed upstream.

groupby_bins:
https://github.com/pydata/xarray/issues/1386
https://github.com/pydata/xarray/pull/1390

test_sel: this is really a pandas bug (fixed in 0.21, but this version 
isn't in Debian yet) but has a workaround in xarray:
https://github.com/pandas-dev/pandas/issues/16896
https://github.com/pydata/xarray/pull/1479

Both fixes are in the 0.10 upstream release, but be aware that this is a 
mildly API-breaking release 
(http://xarray.pydata.org/en/latest/whats-new.html#breaking-changes).