make loky a default on Windows and MacOS but not on Linux

[basnijholt]

Description

Make loky a default on Windows and MacOS but not on Linux.

I am doing this because several people have indicated that they are experiencing issues with Loky on a Linux cluster.
On Windows and MacOS loky is preferred because ProcessPoolExecutor won't work in interactive environments.

Closes #301

See also

• can't pickle lru_cache function with loky #292
• Runner fails in the notebook #299

Checklist

• Fixed style issues using pre-commit run --all (first install using pip install pre-commit)
• pytest passed

Type of change

Check relevant option(s).

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

[akhmerov]

Sounds good to me.

[codecov-io]

Codecov Report

Merging #300 (9e0dad9) into master (75a3274) will increase coverage by 0.08%.
The diff coverage is 70.00%.

@@            Coverage Diff             @@
##           master     #300      +/-   ##
==========================================
+ Coverage   80.61%   80.70%   +0.08%     
==========================================
  Files          35       35              
  Lines        4623     4623              
  Branches      831      832       +1     
==========================================
+ Hits         3727     3731       +4     
+ Misses        771      768       -3     
+ Partials      125      124       -1     

Impacted Files	Coverage Δ
adaptive/runner.py	71.53% <66.66%> (+0.96%)	⬆️
adaptive/tests/test_runner.py	95.00% <71.42%> (+0.06%)	⬆️
adaptive/tests/test_cquad.py	92.25% <0.00%> (+0.64%)	⬆️

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