[skeleton] use python3 flake8
In tox "flake8" environment, use python 3 virtualenv, so we can check for
python3 incompatible syntax.
[tox]
envlist = py27,py3,flake8
[testenv]
deps =
pytest
commands =
{envpython} -m pytest {posargs:test}
[testenv:flake8]
basepython = python3
skip_install = true
deps =
flake8
commands = flake8
[flake8]
exclude = cubicweb_%(cubename)s/migration/*,test/data/*,.tox/*