Use Python 3.6 by default

.travis.yml

@@ -2,7 +2,7 @@ sudo: false
 language: python
 python:
 - 2.7
-- 3.5
+- 3.6
 install:
 - pip install tox "setuptools>=28.2"
 script:
@@ -16,7 +16,7 @@ deploy:
   on:
     tags: true
     all_branches: true
-    python: 3.5
+    python: 3.6
   user: jaraco
   # supply password with `travis encrypt --add deploy.password`
   distributions: dists