Fix tox, take 3
    
    It seems like Flask needs this itsdangerous (seriously) package.
    But there's a bug in a previous version:
    https://github.com/pallets/itsdangerous/issues/102
    Let's update this package to avoid this embarassing problem.
    
    Signed-off-by: Giulia Naponiello <gnaponie@redhat.com>