Support Python 3.7

[blueyed]

Travis does not support 3.7 on the Trusty images: https://docs.travis-ci.com/user/languages/python/#development-releases-support.. :/

[blueyed]

Coverage reporting (coverage xml) fails, which I could not reproduce using Python 3.7.0b1 - Travis is using a4 still.
https://travis-ci.org/pytest-dev/pytest-django/builds/337570117
Will re-try it later.

[codecov-io]

Codecov Report

Merging #576 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #576   +/-   ##
=======================================
  Coverage   92.36%   92.36%           
=======================================
  Files          32       32           
  Lines        1715     1715           
  Branches      142      142           
=======================================
  Hits         1584     1584           
  Misses         94       94           
  Partials       37       37

Flag	Coverage Δ
#dj110	84.48% <ø> (ø)	⬆️
#dj111	86.58% <ø> (ø)	⬆️
#dj18	85.3% <ø> (ø)	⬆️
#dj19	84.37% <ø> (ø)	⬆️
#dj20	84.78% <ø> (ø)	⬆️
#dj21	80.64% <ø> (-4.14%)	⬇️
#mysql_innodb	84.78% <ø> (ø)	⬆️
#mysql_myisam	84.72% <ø> (ø)	⬆️
#postgres	88.16% <ø> (ø)	⬆️
#py27	89.73% <ø> (ø)	⬆️
#py34	84.37% <ø> (ø)	⬆️
#py35	84.48% <ø> (ø)	⬆️
#py36	85.24% <ø> (ø)	⬆️
#py37	80.64% <ø> (?)
#sqlite	86.7% <ø> (+0.11%)	⬆️
#sqlite_file	84.37% <ø> (ø)	⬆️

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