[skeleton] use python3 flake8
In tox "flake8" environment, use python 3 virtualenv, so we can check for
python3 incompatible syntax.
--- a/cubicweb/skeleton/tox.ini.tmpl Tue Jan 29 14:02:19 2019 +0100
+++ b/cubicweb/skeleton/tox.ini.tmpl Tue Jan 29 14:04:28 2019 +0100
@@ -8,6 +8,7 @@
{envpython} -m pytest {posargs:test}
[testenv:flake8]
+basepython = python3
skip_install = true
deps =
flake8