Try with a containor not containing pylint preinstalled, install dev deps (pylint)

.gitlab-ci.yml

@@ -54,8 +54,10 @@ lint:
   stage: checks
   script:
     - poetry config virtualenvs.in-project true
-    - poetry install --no-dev
+    - poetry install
     - make pylint
+  variables:
+    PYTHON_VERSION: "3.9"
 
 .tests:
   extends: .changes