Added mypy to the travis tests

Author: syrusakbary

.travis.yml

@@ -35,8 +35,10 @@ matrix:
   - python: '3.6'
     after_install:
     - pip install pytest-asyncio
+    - pip install mypy
     script:
     - py.test --cov=graphql graphql tests tests_py35
+    - mypy graphql --ignore-missing-imports
   - python: '2.7'
 
 deploy: