[ci] #118: only install dev dependencies

- Try to override the pylint import issues by creating the virtualenv
  inside the project's root directory

.gitlab-ci.yml

@@ -53,6 +53,8 @@ lint:
     - .code_changes
   stage: checks
   script:
+    - poetry config virtualenvs.in-project true
+    - poetry install --no-dev
     - make pylint
 
 .tests: