Python 3.8 compatibility

[roxyboy]

Updated some functions that had deprecation warnings.

[roxyboy]

The tests all pass in Python 3.6-3.8 but fails in Python 2.7 due to the function dask.drop_vars() not being supported. How do you think about dropping support for Python 2.7 @rabernat ?? Otherwise, I can revert back to using dask.drop() but this will be deprecated in future Python 3 releases.

[rabernat]

I think we can drop python 2 in future releases. Thanks for this!

[codecov-io]

Codecov Report

Merging #194 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
+ Coverage   87.99%   88.07%   +0.08%     
==========================================
  Files          12       12              
  Lines        1633     1644      +11     
  Branches      335      337       +2     
==========================================
+ Hits         1437     1448      +11     
  Misses        158      158              
  Partials       38       38

Impacted Files	Coverage Δ
xmitgcm/mds_store.py	92.72% <100%> (+0.06%)	⬆️
xmitgcm/file_utils.py	100% <0%> (ø)	⬆️
xmitgcm/llcreader/known_models.py	78.37% <0%> (+0.6%)	⬆️

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[roxyboy]

Ok, all tests in py27 and py3 pass now :)

[rabernat]

I guess this will also fix the annoying deprecations warnings from xarray.auto_combine. Great work Takaya!