fix: Upgrade flake8 job to py3.7

Author: untitaker

.travis.yml

@@ -13,7 +13,7 @@ matrix:
       sudo: true
 
     - name: Flake8
-      python: "3.6"
+      python: "3.7"
       install:
         - pip install tox
       script: tox -e flake8