Use real python 3.7 from travis

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

.travis.yml

@@ -6,7 +6,11 @@ python:
   - "3.3"
   - "3.5"
   - "3.6"
-  - "3.7-dev"
+matrix:
+  include:
+    - python: 3.7
+      dist: xenial
+      sudo: true
 # command to install dependencies, e.g. pip install -r requirements.txt --use-mirrors
 install:
     - 'pip install -e ".[test]"'