[skeleton] use python3 flake8 In tox "flake8" environment, use python 3 virtualenv, so we can check for python3 incompatible syntax.

mock
psycopg2
ldap3 < 2
cubicweb-basket
cubicweb-card
cubicweb-comment
cubicweb-file
cubicweb-localperms
cubicweb-tag