Add testing and document support for Python 3.7

[jdufresne]

Python 3.7 was released on June 27, 2018.

[codecov]

Codecov Report

Merging #1068 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1068   +/-   ##
=======================================
  Coverage   84.08%   84.08%           
=======================================
  Files          24       24           
  Lines        1301     1301           
  Branches      178      178           
=======================================
  Hits         1094     1094           
  Misses        157      157           
  Partials       50       50

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[matthiask]

Awesome, thanks!

[adamchainz]

@jdufresne just noticed use of xenial in the travis file whilst editing for #1109 . I can't find any Travis documentation about the xenial environment, how did you know it exists?

[jdufresne]

@adamchainz Best I found is travis-ci/travis-ci#9815 which has some explanation why xenial is a necessary workaround for Python 3. 7

[adamchainz]

¯_(ツ)_/¯ okay, will give it a try on my projects