Use the latest Python for testing

Now, the tests are explicitly executed for Python versions 2.7, 3.7, 3.8, 3.9 and the latest version, which is 3.10 at the moment.

.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:3.6
+image: ${CI_DEPENDENCY_PROXY_DIRECT_GROUP_IMAGE_PREFIX}/python:latest
 
 # Change pip's cache directory to be inside the project directory since we can
 # only cache local items.