drop support for py3.3 and include 3.7

Author: sideffect0

.travis.yml

@@ -1,10 +1,10 @@
 language: python
 python:
   - "2.7"
-  - "3.3"
   - "3.4"
   - "3.5"
   - "3.6"
+  - "3.7"
   - "pypy"
 install: "pip install -r dev-requirements.txt"
 script: python setup.py test