[IMP][MIG] lunch module : migrare to new api and lint a module

- Task: https://www.odoo.com/web?#id=10063&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.ap7PWdIPg6gfU334
actually, this onchange is not required anymore as `total` is a computed field; you only need to add the correct `@depends` dependencies on the compute function (which is not correct).

so merge this code into the compute function