Test: update tox config, add flake8 section

tox.ini

 [tox]
-envlist = py27, py3
+envlist = py27, py3, flake8
 
 [testenv]
 deps =
   pytest
-
 commands =
   {envpython} -m pytest {posargs}
 
@@ -12,3 +11,14 @@ commands =
 python_files = *test_*.py
 testpaths = test
 addopts = -r fEs
+
+[testenv:flake8]
+basepython = python3
+skip_install = true
+deps =
+  flake8
+commands = {envpython} -m flake8 {toxinidir} {posargs}
+
+[flake8]
+max-line-length = 120
+exclude = cubicweb_person/migration/*,.tox/*