Migrate CircleCI docker image (#1767)

CCI says circleci/python:3.8 is deprecaetd. Migrating to cimg/python:3.8.

> CircleCI’s latest pre-built container images were designed from the ground up to help your team build more reliably.
> Our new images were built specifically for continuous integration projects and they are our most deterministic, performant, and efficient images yet.

> As of Dec 31, 2021, legacy images will no longer be supported on CircleCI.

Related links

- https://circleci.com/blog/announcing-our-next-generation-convenience-images-smaller-faster-more-deterministic
- https://discuss.circleci.com/t/legacy-convenience-image-deprecation/41034
- https://circleci.com/docs/2.0/next-gen-migration-guide

Author: mthrok

.circleci/config.yml

@@ -105,7 +105,7 @@ smoke_test_common: &smoke_test_common
 jobs:
   circleci_consistency:
     docker:
-      - image: circleci/python:3.8
+      - image: cimg/python:3.8
     steps:
       - checkout
       - run:
@@ -298,7 +298,7 @@ jobs:
         description: "What whl subfolder to upload to, e.g., blank or cu100/ (trailing slash is important)"
         type: string
     docker:
-      - image: circleci/python:3.8
+      - image: cimg/python:3.8
     steps:
       - attach_workspace:
           at: ~/workspace
@@ -633,7 +633,7 @@ jobs:
   docstring_parameters_sync:
     <<: *binary_common
     docker:
-      - image: circleci/python:3.8
+      - image: cimg/python:3.8
     steps:
       - checkout
       - run: