[FIX] purchase_requisition: state_blanket_order on multiple records

Reading `state_blanket_order` on multiple records was failing.

e.g.
```python
env['purchase.requisition'].create({})
env['purchase.requisition'].create({})
env['purchase.requisition'].search([]).mapped('state_blanket_order')
```
raised
```python
ValueError: Expected singleton: purchase.requisition(5, 1, 4)
```

closes odoo#48639

X-original-commit: 031c3f3
Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>

addons/purchase_requisition/models/purchase_requisition.py

@@ -71,7 +71,8 @@ def _get_type_id(self):
 
     @api.depends('state')
     def _set_state(self):
-        self.state_blanket_order = self.state
+        for requisition in self:
+            requisition.state_blanket_order = requisition.state
 
     @api.onchange('vendor_id')
     def _onchange_vendor(self):