update circleci image (#392)

Author: juagargi

.circleci/config.yml

@@ -72,7 +72,7 @@ commands:
 jobs:
   style-check:
     docker:
-      - image: circleci/python:3.6.13
+      - image: cimg/python:3.6.13
     steps:
       - install_git_lfs
       - cached_checkout
@@ -91,7 +91,7 @@ jobs:
 
   unit-tests:
     docker:
-      - image: circleci/python:3.6.13
+      - image: cimg/python:3.6.13
     steps:
       - install_git_lfs
       - cached_checkout