Upgrade python in CI (#18)

Author: rupert

.circleci/config.yml

@@ -3,7 +3,7 @@ jobs:
   lint:
     working_directory: ~/project
     docker:
-      - image: circleci/python:3.6.5-stretch
+      - image: circleci/python:3.7.2-stretch
     steps:
       - checkout
       - run: |
@@ -17,7 +17,7 @@ jobs:
   test:
     working_directory: ~/project
     docker:
-      - image: circleci/python:3.6.5-stretch
+      - image: circleci/python:3.7.2-stretch
     steps:
       - checkout
       - run: |