[skeleton] drop whitelist_externals for flake8
whitelist_externals makes sense when used with sitepackages=true.
In this case it will always rely on $VIRTUAL_ENV/bin/flake8 so, the
whitelist_externals has no effect.
[tox]
envlist = py27,py3,flake8
[testenv]
deps =
pytest
commands =
{envpython} -m pytest {posargs:test}
[testenv:flake8]
skip_install = true
deps =
flake8
commands = flake8
[flake8]
exclude = cubicweb_%(cubename)s/migration/*,test/data/*,.tox/*