ci(project): pin to Python 3.6.8

3.6.9 image contains Node 12 which is incompatible with JSDoc

Author: kylef

.circleci/config.yml

@@ -44,7 +44,7 @@ jobs:
   docs:
     <<: *defaults
     docker:
-      - image: circleci/python:3.6-node-browsers
+      - image: circleci/python:3.6.8-node-browsers
     steps:
       - checkout
       - <<: *install