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.

[MASTER]
load-plugins=cubicweb.pylintext
ignore=__pkginfo__

[MESSAGES CONTROL]
disable = too-many-ancestors,too-many-instance-attributes,too-many-public-methods,
      too-few-public-methods,too-many-arguments,import-error

[BASIC]
function-rgx = [a-z_][a-z0-9_]{2,35}$
good-names=w,_

[TYPECHECK]
generated-members=debug,info,notice,warning,error,critical,exception

[CLASSES]
exclude-protected=_cw,_cnx,
      # namedtuple public API.
      _asdict,_fields,_replace,_source,_make


[FORMAT]
max-line-length=100
max-module-lines=2000