Merge pull request #3621 from Carreau/test-3.7

start testing on 3.7

Author: takluyver

.travis.yml

@@ -96,6 +96,8 @@ matrix:
           env: GROUP=python
         - python: 3.5
           env: GROUP=python
+        - python: "3.7-dev"
+          env: GROUP=python
         - python: 3.6
           env: GROUP=docs