[feature] add stylecheck to CI

flake8 is an automatic style checker. This is very useful to ensure that
we have a consistent style as more people start contributing to the
code.

We ignore coolr for now, as it's a big offender.

.gitlab-ci.yml

 stages:
   - test
+  - style
 
 py.test:
   stage: test
   script:
     - pipenv install --dev
     - pipenv run py.test
+
+flake8:
+  stage: style
+  script:
+    - pipenv install --dev
+    - pipenv run flake8

Pipfile

@@ -8,6 +8,7 @@ name = "pypi"
 
 [dev-packages]
 pytest = "*"
+"flake8" = "*"
 
 [requires]
 python_version = "2.7"

setup.cfg

+[flake8]
+exclude = .git, doc, nrm/coolr