Use pylint in merge requests, only check the changed '.py' files in this MR.

.gitlab-ci.yml

stages:
  - test

workflow:
  rules:
    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'

pylint-error-only:
  stage: test
  script:
    - pylint -E `git ls-files '*.py'`

pylint-mr:
  stage: test
  script:
    - FLIST=$(git --no-pager diff --name-only $(git merge-base --fork-point origin/$CI_MERGE_REQUEST_TARGET_BRANCH_NAME) $CI_COMMIT_SHA | grep "\.py$")
    - echo $FLIST | xargs -r pylint -f parseable -r y