CI: Update convenience images

Author: effigies

.circleci/config.yml

@@ -426,7 +426,7 @@ jobs:
 
   test_package:
     docker:
-      - image: circleci/python:3.8.5
+      - image: cimg/python:3.9
     working_directory: /tmp/src/niworkflows
     steps:
       - checkout
@@ -450,7 +450,7 @@ jobs:
 
   deploy_pypi:
     docker:
-      - image: circleci/python:3.8.5
+      - image: cimg/python:3.9
     working_directory: /tmp/src/niworkflows
     steps:
       - checkout
@@ -525,7 +525,7 @@ jobs:
 
   build_docs:
     docker:
-      - image: circleci/python:3.8.5
+      - image: cimg/python:3.9
     environment:
       - FSLOUTPUTTYPE: NIFTI
       - SUBJECTS_DIR: /tmp/subjects