Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -5,7 +5,7 @@
 
 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/python/tags/
-image: python:latest
+image: python:3.9
 
 # Change pip's cache directory to be inside the project directory since we can
 # only cache local items.