Downgrade ci machine to debian stretch due to ssl3 errors

.gitlab-ci.yml

 # This file is a template, and might need editing before it works on your project.
 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/python/tags/
-image: python:3-buster
+image: python:3-stretch
 
 # Change pip's cache directory to be inside the project directory since we can
 # only cache local items.