Update python Docker tag to v3.9

This MR contains the following updates:

Package	Type	Update	Change
python	image	minor	3.8-buster -> 3.9-buster

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.gitlab-ci.yml

 test:
-  image: python:3.8-buster
+  image: python:3.9-buster
   before_script:
     - apt-get update -y -qq && apt-get install -y -qq yarnpkg
     - pip install -q Django