fix py36 xarray master build

[timothyas]

This (seems to) fix the current bug with the python 3.6, xarray master build. It seems that the newest version of xarray.combine_by_coords added the option combine_attrs, allowing the user to specify how to handle attributes while combining multiple datasets. The default is no_conflicts, which in this case causes a problem with the history attribute since each dataset corresponds to an iteration (or None for the grid), each of which is different and shows up in this attribute as "created by calling open_mdsdataset(.....)"

It seems that the old default was to simply drop attributes, so that's what I'm setting the option to for a first pass. We could also just drop the attributes before calling xr.combine_by_coords, and remove the need for testing.

[codecov-io]

Codecov Report

Merging #200 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
+ Coverage   88.07%   88.09%   +0.01%     
==========================================
  Files          12       12              
  Lines        1644     1646       +2     
  Branches      337      338       +1     
==========================================
+ Hits         1448     1450       +2     
  Misses        158      158              
  Partials       38       38              

Impacted Files	Coverage Δ
xmitgcm/mds_store.py	92.76% <100.00%> (+0.03%)	⬆️

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[rabernat]

Good catch Tim! This was added quite recently in pydata/xarray#3877.

Fortunately our CI is testing against xarray master from github, which is how we caught the incompatibility. That's how it's supposed to work!