[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.

### Requirements for tests in various cubicweb/**/test directories. ###

## shared by several test folders
cubicweb-card
docutils
Twisted < 16.0.0
webtest

## cubicweb/test
Pygments
mock
#fyzz XXX pip install fails
cubicweb-file
cubicweb-localperms
cubicweb-tag

## cubicweb/devtools/test
flake8

## cubicweb/hooks/test
psycopg2

## cubicweb/pyramid/test
pyramid >= 1.5.0
waitress >= 0.8.9
wsgicors >= 0.3
pyramid_multiauth

## cubicweb/sobject/test
cubicweb-comment