Do not use deepcopy on models

src/pretix/base/services/orders.py

@@ -1,4 +1,3 @@
-import copy
 import json
 import logging
 from collections import Counter, namedtuple
@@ -44,6 +43,7 @@ from pretix.base.signals import (
     allow_ticket_download, order_fee_calculation, order_placed, periodic_task,
 )
 from pretix.celery_app import app
+from pretix.helpers.models import modelcopy
 from pretix.multidomain.urlreverse import build_absolute_uri
 
 error_messages = {
@@ -923,7 +923,7 @@ class OrderChangeManager:
             op.save()
 
         try:
-            ia = copy.deepcopy(self.order.invoice_address)
+            ia = modelcopy(self.order.invoice_address)
             ia.pk = None
             ia.order = split_order
             ia.save()
@@ -947,7 +947,7 @@ class OrderChangeManager:
             split_order.total += fee.value
 
         for fee in self.order.fees.exclude(fee_type=OrderFee.FEE_TYPE_PAYMENT):
-            new_fee = copy.deepcopy(fee)
+            new_fee = modelcopy(fee)
             new_fee.pk = None
             new_fee.order = split_order
             split_order.total += new_fee.value