fix(CI): Use the proper image for tox command

Change the image used for the tox -e py3 since the registry r.ext.logilab.fr is not available any more

.gitlab-ci.yml

@@ -20,7 +20,7 @@ flake8:
 
 py3:
   stage: tests
-  image: r.ext.logilab.fr/jenkins/jnlp:buster
+  image: ${CI_REGISTRY}/cubicweb/dockerfiles/buster-slim-pg11-ldap
   script: tox -e py3
   artifacts:
     paths: