v4: Add support for Python 3.7

[hugovk]

Like #615 but for the v4 branch.

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Add support for Python 3.7
• A small workaround is needed for Travis CI, see Add Python 3.7 option travis-ci/travis-ci#9815

[codecov]

Codecov Report

Merging #637 into v4 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##               v4     #637   +/-   ##
=======================================
  Coverage   84.38%   84.38%           
=======================================
  Files          37       37           
  Lines         967      967           
  Branches      108      108           
=======================================
  Hits          816      816           
  Misses         93       93           
  Partials       58       58

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[thinkingserious]

Hello @hugovk,

Thanks again for the PR!

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