Work around changes in xarray 0.15

[mraspaud]

Xarray v0.15.0 brings some changes that broke the satpy unit tests.
Here we provide a fix for coarsen and use drop_vars in instead of del:
pydata/xarray#3746

• Tests passed
• Passes flake8 satpy

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1c4c43c). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1058   +/-   ##
=========================================
  Coverage          ?   88.09%           
=========================================
  Files             ?      191           
  Lines             ?    29438           
  Branches          ?        0           
=========================================
  Hits              ?    25934           
  Misses            ?     3504           
  Partials          ?        0

Impacted Files	Coverage Δ
satpy/tests/compositor_tests/__init__.py	99.61% <100%> (ø)
satpy/readers/abi_base.py	91.79% <100%> (ø)
satpy/readers/scmi.py	82.91% <100%> (ø)
satpy/scene.py	89.91% <100%> (ø)

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[mraspaud]

I think this should be squash-merged

[djhoese]

If this passes tests, is it ready for merge?

Based on the changes, I assume you searched for del ...coords['coord'] usage?

[mraspaud]

Yes, and yes.