[WIP] Support Python 3.7 in Cloudpickle

[HyukjinKwon]

This PR targets to support Python 3.7 in Cloudpickle.

[HyukjinKwon]

Currently blocked by https://github.com/travis-ci/travis-ci/issues/9552.

[HyukjinKwon]

See also travis-ci/travis-ci#9815

[jakirkham]

Maybe we can start installing Miniconda and create different versioned Python environments for each matrix element?

[HyukjinKwon]

Looks there are workarounds for it, for instance, travis-ci/travis-ci#9815 (comment) . I don't think this is super urgent and we could just wait for its support though.

[codecov-io]

Codecov Report

Merging #181 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #181   +/-   ##
=======================================
  Coverage   83.85%   83.85%           
=======================================
  Files           1        1           
  Lines         539      539           
  Branches       95       95           
=======================================
  Hits          452      452           
  Misses         63       63           
  Partials       24       24

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[tomMoral]

I am currently working on python3.7 support with #189.
I think you can close this one.

[HyukjinKwon]

Yea, thx.