Skip shell completion test entirely on Windows

[jwodder]

It just doesn't work on conda-forge.

[codecov]

Codecov Report

Merging #702 (362887e) into master (31cf2e5) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #702      +/-   ##
==========================================
+ Coverage   83.99%   84.06%   +0.06%     
==========================================
  Files          59       59              
  Lines        5668     5665       -3     
==========================================
+ Hits         4761     4762       +1     
+ Misses        907      903       -4     

Flag	Coverage Δ
unittests	84.06% <100.00%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
dandi/cli/tests/test_shell_completion.py	100.00% <100.00%> (+9.09%)	⬆️
dandi/dandiapi.py	90.37% <0.00%> (-0.54%)	⬇️
dandi/_version.py	45.96% <0.00%> (+1.75%)	⬆️

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[yarikoptic]

ok, let's please conda's anti-windows Gods and try with new release