[gitlab] Add a minimal file for Continuous Integration

.gitlab-ci.yml

+image: python:latest
+
+stages:
+  - lint
+
+lint_py:
+  stage: lint
+  before_script:
+    - pip install tox
+  script:
+    - tox -e flake8
+  only:
+    changes:
+      - "*.py"