Travis requires a workaround for Python 3.7

Author: rayluo

.travis.yml

@@ -4,7 +4,14 @@ python:
   - "2.7"
   - "3.5"
   - "3.6"
-  - "3.7"
+# Borrowed from https://github.com/travis-ci/travis-ci/issues/9815
+# Enable 3.7 without globally enabling sudo and dist: xenial for other build jobs
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
+
 install:
   - pip install -r requirements.txt
 script: