[skeleton] Don't use system site packages in tox environment As we changed this in cubicweb, there is a consensus that this is not the best practice so let us change it as well for skeleton.

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