[IMP] purchase : Improve purchase module views - BST onboarding

https://www.odoo.com/web#id=10537&view_type=form&model=project.task&menu_id=3942&action=327
pad: https://pad.odoo.com/p/r.0g7NjI7wwVRcZBem
`for wizard in self.browse(ids):
       for partner_id in wizard.parnter_ids:
                make_purchase_order()`
Extract `self.pool.get('purchase.requisition')` in a variable before the loops.