Non writtable fields are not readonly
From https://discuss.tryton.org/t/field-access-rights-to-a-specific-group/1787
Since r 1094d993d89c, the test to make fields readonly is wrong in Model.fields_get()
From https://discuss.tryton.org/t/field-access-rights-to-a-specific-group/1787
Since r 1094d993d89c, the test to make fields readonly is wrong in Model.fields_get()