WIP fix for 821: test spectral smooth on windows

[keflavich]

This is a WIP: no fix has been put in place, I just made the tests more granular so we can see where the failure occurs.

Sidenote: It was extremely difficult to set up the testing environment to enable me to debug this. I had to copy-paste line-by-line code from conftest.py and test_regrid to get the cubes put in place and then loaded. This is an immense headache and makes me hate working with pytest. Is there any sane way to interactively debug pytest-based functions?

[codecov-commenter]

Codecov Report

Merging #823 (30de5ea) into master (b4ff491) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
+ Coverage   77.93%   77.95%   +0.01%     
==========================================
  Files          24       24              
  Lines        5861     5861              
==========================================
+ Hits         4568     4569       +1     
+ Misses       1293     1292       -1     

Impacted Files	Coverage Δ
spectral_cube/cube_utils.py	82.59% <0.00%> (+0.31%)	⬆️

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[keflavich]

This is an upstream error; the kernel is getting populated differently.

[keflavich]

OK this isn't an error at all. Our test needs fixing.

astropy/astropy#13299

[keflavich]

I had to rebase this because I branched off the wrong branch, but tests passed; it should be good to go though.

On my local branch, I get an error:
https://github.com/keflavich/spectral-cube/runs/6852738789?check_suite_focus=true#step:5:124
that is caused by an outdated doctestplus. I don't know why that affects me differently from master. There may be no solution if this is a real error, though, as it affects python3.6, which is no longer supported by pretty much anyone.