gitlab-ci: Use 'tox' to run 'flake8'

Signed-off-by: Vasilis Tsiligiannis <acinonyx@openwrt.gr>

.gitlab-ci.yml

@@ -14,10 +14,10 @@ static_js_css:
 static_python:
   stage: static
   image: python:2.7
+  before_script:
+    - pip install tox
   script:
-    - pip install --no-cache-dir --no-deps -r requirements-dev.txt
-    - cp env-dist .env
-    - flake8 .
+    - tox -e flake8
 build:
   stage: build
   image: python:2.7