Workaround issue with relpath on Windows

[ogrisel]

Fix for #895.

Writing a non-regression test for this is going to be tricky. It might be possible using https://stackoverflow.com/a/827398/163740 to see if there is another drive / mount available on the CI machine and chdir to it before writing the coverage file but it's complicated.

Let me know if you want me to try to write such a non-regression test or not.

Besides, I have tested this branch on the scikit-learn CI scikit-learn/scikit-learn#15909 and confirms that this fixes the original problem.

[codecov-io]

Codecov Report

Merging #896 into master will increase coverage by 0.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #896      +/-   ##
==========================================
+ Coverage    94.2%   94.22%   +0.01%     
==========================================
  Files          86       86              
  Lines       11852    11870      +18     
  Branches     1182     1185       +3     
==========================================
+ Hits        11165    11184      +19     
+ Misses        560      558       -2     
- Partials      127      128       +1

Impacted Files	Coverage Δ
coverage/sqldata.py	92.93% <50%> (-0.4%)	⬇️
tests/helpers.py	100% <0%> (ø)	⬆️
coverage/control.py	91.68% <0%> (+0.02%)	⬆️
tests/test_concurrency.py	96.33% <0%> (+0.21%)	⬆️
coverage/inorout.py	72.91% <0%> (+0.41%)	⬆️
tests/conftest.py	96.42% <0%> (+7.14%)	⬆️

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[ogrisel]

The travis failure is:

worker 'gw2' crashed while running 'tests/test_oddball.py::RecursionTest::test_long_recursion'

It seems unrelated to this PR.

[ogrisel]

@nedbat please let me know if you need anything else from my side.

[nedbat]

Looks great, thanks. I'll amend the commit to add changelog and bug number info.

[ogrisel]

Cheers!