add support for loky

[basnijholt]

Description

This adds support for loky which has a ProcessPoolExecutor that can for example use cloudpickle.

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

[codecov-io]

Codecov Report

Merging #263 into master will increase coverage by 0.02%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
+ Coverage   79.42%   79.44%   +0.02%     
==========================================
  Files          32       32              
  Lines        4403     4423      +20     
  Branches      811      815       +4     
==========================================
+ Hits         3497     3514      +17     
- Misses        778      781       +3     
  Partials      128      128              

Impacted Files	Coverage Δ
adaptive/runner.py	65.18% <64.70%> (-0.58%)	⬇️
adaptive/tests/test_runner.py	92.20% <100.00%> (+0.77%)	⬆️
adaptive/learner/learner2D.py	78.38% <0.00%> (+0.36%)	⬆️
adaptive/learner/learner1D.py	92.33% <0.00%> (+0.61%)	⬆️

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[jbweston]

LGTM modulo test failures

[basnijholt]

I've opened joblib/loky#240.

[basnijholt]

@jbweston, I think the remaining failures are related to https://bugs.python.org/issue36281 and #156.

Not sure how to tackle these.

I've opened joblib/loky#241.