Fix broken flake8 configuration and flake8 errors which were hidden by this breakage. flake8 --filename options doesn't work as expected: * it's expected to be a shell pattern, using stdlib's fnmatch.fnmatch function internally. This funciton thinks that 'cubicweb/x.py' doesn't match 'cubicweb/x.py' (there must be a reason but that's not the point), hence no file was actually checked ; * as this is a list of pattern, each encountered file is checked against each pattern, leading to run time explosion. So maintain list of files to check in a separated file and give this list to flake8 using unix's xarg command.

^build$
^dist$
\.egg-info$
^.tox$
^.cache$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$
\.old$
syntax: regexp
.*/data.*/database/.*
.*/data/ldapdb/.*
.*/data/uicache/
.*/data/libpython/cubicweb_.*/i18n/.*\.po
^doc/html/
^doc/doctrees/
^doc/book/en/devweb/js_api/
^doc/_build
^doc/js_api/
test-results.xml