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.