Update Circle CI base image (#7518)

[...] 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.

.circleci/config.yml

@@ -3,7 +3,7 @@ version: 2
 jobs:
   backend:
     docker:
-      - image: circleci/python:3.7.3
+      - image: cimg/python:3.8.11
     steps:
       - checkout
       - restore_cache:
@@ -50,7 +50,7 @@ jobs:
 
   nightly-build:
     docker:
-      - image: circleci/python:3.7.3
+      - image: cimg/python:3.8.11
     steps:
       - checkout
       - run: cd ~ && wget https://nodejs.org/dist/v14.17.5/node-v14.17.5-linux-x64.tar.gz