Add workaround to make Python3.7 in Travis work properly

.travis.yml

@@ -4,7 +4,13 @@ python:
 - '3.4'
 - '3.5'
 - '3.6'
-- '3.7'
+# Enable 3.7 without globally enabling sudo and dist: xenial for other build jobs
+# See https://github.com/travis-ci/travis-ci/issues/9815
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
 sudo: false
 cache: pip
 install: