Run flake8 on Travis CI and tox

.travis.yml

@@ -12,6 +12,10 @@ matrix:
         - python: "3.7"
           dist: xenial
           sudo: true
+        - name: "flake8"
+          install: pip install flake8
+          script: flake8 src setup.py
+          after_success:
 install:
     - pip install -U pip setuptools
     - pip install -U coverage coveralls

setup.cfg

@@ -1,2 +1,5 @@
 [bdist_wheel]
 universal = 1
+
+[flake8]
+doctests = yes

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist =
-   py27,py34,py35,py36,py37,pypy,pypy3,coverage,docs
+   flake8,py27,py34,py35,py36,py37,pypy,pypy3,coverage,docs
 
 [testenv]
 commands =
@@ -24,3 +24,8 @@ commands =
     sphinx-build -b html -d docs/_build/doctrees docs docs/_build/html
 deps =
     .[docs]
+
+[testenv:flake8]
+deps = flake8
+skipinstall = true
+commands = flake8 setup.py src