[tox] Drop "exclude" option from flake8 config and adjust flake8-ok-files.txt accordingly Some files listed in flake8-ok-files.txt were excluded of flake8 run because of the "exclude" option in tox.ini. Some of them were non-existent files (moved), some others were actually non-flake8-compliant. In the latter case, we adjust trivial errors (blank lines, module import not on top of file) but remove others (like cubicweb/test/unittest_utils.py) from flake8-ok-files.txt.