[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.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)