Master website event sale seats availability psa tde

- Task: https://www.odoo.com/web?#id=9971&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.BTJ9yFYnbxBG3VzQ
The internal listcomp is redundant (the gencomp can be fed directly to `set`).

Not entirely sure the list is necessary either (but I'm uncertain on that)