Set lines readonly if no company is set
As https://discuss.tryton.org/t/problem-with-sale-price-list-after-migration-to-6-0/4352 shown, the lines need to have a company to be edited once the price list are used.
As https://discuss.tryton.org/t/problem-with-sale-price-list-after-migration-to-6-0/4352 shown, the lines need to have a company to be edited once the price list are used.