Update .gitlab-ci.yml file

.gitlab-ci.yml

 # Official language image. Look for the different tagged releases at:
 # https://hub.docker.com/r/library/python/tags/
-image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.7
+image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:3.6
 
 # Change pip's cache directory to be inside the project directory since we can
 # only cache local items.