ci: use buster image for py27 tests

.gitlab-ci.yml

@@ -10,7 +10,7 @@ flake8:
   script: tox -e flake8
 
 py27:
-  image: r.ext.logilab.fr/jenkins/jnlp:buster
+  image: registry.logilab.fr/logilab/infra/dockerfiles/debian-buster-slim-pg11
   # before_script:
   #   - sudo pip install tox
   #   - sudo apt-get update