[FIX] sale_invoice_policy: invoice_policy

Allow empty value on invoice_policy by removing readonly option.
Without this it's not possible to select nothing which forces always to
use invoice policy from sale order level.

Author: Drsmartinkus

sale_invoice_policy/models/sale_order.py

@@ -8,8 +8,10 @@ class SaleOrder(models.Model):
     _inherit = "sale.order"
 
     invoice_policy = fields.Selection(
-        [("order", "Ordered quantities"), ("delivery", "Delivered quantities")],
-        readonly=True,
+        [
+            ("order", "Ordered quantities"),
+            ("delivery", "Delivered quantities")
+        ],
         help="Ordered Quantity: Invoice based on the quantity the customer "
         "ordered.\n"
         "Delivered Quantity: Invoiced based on the quantity the vendor "