travisci: Test Python 3.7 as well

Use the workaround from

    travis-ci/travis-ci#9815

since Python 3.7 is not yet supported by the default image.

Author: agx

.travis.yml

@@ -9,6 +9,11 @@ python:
   - "3.4"
   - "3.5"
   - "3.6"
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
 
 before_install:
   - sudo apt-get -qq update