tox / travis: enable testing on py37

(cherry picked from commit fef9581690dd49f9d812a8841955fcdb5fd6ba71)

.travis.yml

@@ -29,6 +29,10 @@ matrix:
           os: linux
           dist: trusty
           env: TOXENV=py36
+        - python: "3.7"
+          os: linux
+          dist: xenial
+          env: TOXENV=py37
         - language: generic
           os: osx
           osx_image: xcode8.3

tox.ini

@@ -2,7 +2,7 @@
 # fakeroot -u tox --recreate
 
 [tox]
-envlist = py{34,35,36},flake8
+envlist = py{34,35,36,37},flake8
 
 [testenv]
 deps =