Travis CI: Add Python 3.7 to the testing

Author: cclauss

.travis.yml

@@ -4,6 +4,12 @@ python:
   - 3.5
   - 3.6
 
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial     # required for Python 3.7 (travis-ci/travis-ci#9069)
+      sudo: required   # required for Python 3.7 (travis-ci/travis-ci#9069)
+
 install:
   - pip install -r requirements_dev.txt