test: integrate flake8 into tox.ini and gitlab-ci

.gitlab-ci.yml

@@ -23,6 +23,10 @@ black:
   stage: tests
   script: tox -e black
 
+flake8:
+  stage: tests
+  script: tox -e flake8
+
 trigger-other-pipelines:
   only:
     refs:

tox.ini

@@ -41,3 +41,15 @@ skip_install = true
 deps =
   black >= 19.10b0
 commands = black .
+
+[testenv:flake8]
+skip_install = true
+deps =
+  flake8 >= 3.6
+commands = flake8
+
+[flake8]
+format = pylint
+ignore = W503, E203, E731, E231
+max-line-length = 100
+exclude = docs/*,.tox/*