Allow scikit-image >= 0.16, fix related adaptive histogram test

[hakonanes]

Signed-off-by: Håkon Wiik Ånes hwaanes@gmail.com

Description

The test kikuchipy/pattern/tests/test_chunk.py::TestAdaptiveHistogramEqualizationChunk::test_adaptive_histogram_equalization_chunk failed with scikit-image 0.17.2, while passed for 0.16.2.

This PR:

• Allow scikit-image >= 0.16 in setup.py
• Ensure mentioned test pass for both scikit-image versions 0.16 and 0.17. This fix leads to a coverage reduction of one line when running the test suite with scikit-image 0.16... But that is acceptable.

For reviewers

• 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
  the unreleased section in doc/changelog.rst.

[coveralls]

Coverage remained the same at 100.0% when pulling 8556c65 on hakonanes:fix-adaptive-histogram-test-skimage into 778da53 on kikuchipy:master.