Try a workaround to make build work on 3.7

https://github.com/travis-ci/travis-ci/issues/9815

.travis.yml

@@ -1,14 +1,22 @@
 language: python
 
 python:
-  - "3.7"
+  - "3.6"
+
+# Enable 3.7 without globally enabling sudo and dist: xenial for other build jobs
+matrix:
+  include:
+    - name: "Python: 3.7"
+      python: "3.7"
+      dist: xenial
+      sudo: true
+      env: TOX_ENV=py37
 
 env:
   - TOX_ENV=py27
   - TOX_ENV=py34
 # - TOX_ENV=py35
   - TOX_ENV=py36
-  - TOX_ENV=py37
 
 before_install:
   # work around https://github.com/travis-ci/travis-ci/issues/8363