backport: _marching_cubes_lewiner_cy: mark char as signed #3587

[hmaarrfk]

Fix marching_cubes_lewiner on ppc64le

Closes gh-2982.

Description

[Tell us about your new feature, improvement, or fix! If relevant, please supplement the PR with images.]

Checklist

[It's fine to submit PRs which are a work in progress! But before they are merged, all PRs should provide:]

• Clean style in the spirit of PEP8
• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests

[For detailed information on these and other aspects see scikit-image contribution guidelines]

References

[If this is a bug-fix or enhancement, it closes issue # ]
[If this is a new feature, it implements the following paper: ]

For reviewers

(Don't remove the checklist below.)

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
• Consider backporting the PR with @meeseeksdev backport to v0.14.x

[soupault]

@hmaarrfk Could you update your branch and rebase, please? The Travis builds are beyond red 😄.

[codecov-io]

Codecov Report

Merging #3678 into v0.14.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           v0.14.x    #3678   +/-   ##
========================================
  Coverage    86.57%   86.57%           
========================================
  Files          337      337           
  Lines        27570    27570           
========================================
  Hits         23868    23868           
  Misses        3702     3702

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[hmaarrfk]

done!

[soupault]

... Now it's failing because of Dask. At least, something is green.