[tox] Bound flake8 version to >=3.4, <3.5
The code base is not clean with 3.5, so keep using 3.4 in *stable*
branches.
missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/*/cubicweb/skeleton/debian/rules)embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js