[16.0][FIX] purchase_security: specify @api.onchange

[BurkhalterY]

Specify @api.onchange on onchange_partner_id(), because without it, the native onchange isn't triggered (I don't know why) and some fields like fiscal_position_id and payment_term_id are never updated.

Others modules of purchase-workflow inherit onchange_partner_id() and set its onchange like this: @api.onchange("partner_id") Since the native one is: @api.onchange('partner_id', 'company_id') I honestly don't know why (excepted for "purchase_order_type", which divide them between two methods).

This issue is reproducible on runboat, but you need to use the "baseonly" database. If you install some modules like "purchase_partner_incoterm", it will work again.

I'm open to any better way to solve this issue.