Allow restore_db_instance_to_point_in_time in custom subnet

Previously, this failed with
    TypeError: cannot pickle 'generator' object
when the code tried to deepcopy the `backend`.

Merge it with the existing code that removed "db_subnet_group".

moto/rds/models.py

@@ -2065,14 +2065,14 @@ def restore_db_instance_to_point_in_time(
             db_instance_identifier=source_db_identifier
         )[0]
 
-        # remove the db subnet group as it cannot be copied
-        # and is not used in the restored instance
-        source_dict = db_instance.__dict__
-        if "db_subnet_group" in source_dict:
-            del source_dict["db_subnet_group"]
-
-        new_instance_props = copy.deepcopy(source_dict)
-        new_instance_props.pop("backend")
+        new_instance_props = {}
+        for key, value in db_instance.__dict__.items():
+            # Remove backend / db subnet group as they cannot be copied
+            # and are not used in the restored instance.
+            if key in ("backend", "db_subnet_group"):
+                continue
+            new_instance_props[key] = copy.deepcopy(value)
+
         if not db_instance.option_group_supplied:
             # If the option group is not supplied originally, the 'option_group_name' will receive a default value
             # Force this reconstruction, and prevent any validation on the default value

tests/test_rds/test_rds.py

@@ -1424,9 +1424,18 @@ def test_restore_db_instance_from_db_snapshot(
     )
 
 
+@pytest.mark.parametrize(
+    "custom_db_subnet_group", [True, False], ids=("custom_subnet", "default_subnet")
+)
 @mock_aws
-def test_restore_db_instance_to_point_in_time():
+def test_restore_db_instance_to_point_in_time(custom_db_subnet_group: bool):
     conn = boto3.client("rds", region_name=DEFAULT_REGION)
+
+    if custom_db_subnet_group:
+        extra_kwargs = {"DBSubnetGroupName": create_db_subnet_group()}
+    else:
+        extra_kwargs = {}
+
     conn.create_db_instance(
         DBInstanceIdentifier="db-primary-1",
         AllocatedStorage=10,
@@ -1436,6 +1445,7 @@ def test_restore_db_instance_to_point_in_time():
         MasterUsername="root",
         MasterUserPassword="hunter2",
         DBSecurityGroups=["my_sg"],
+        **extra_kwargs,
     )
     assert len(conn.describe_db_instances()["DBInstances"]) == 1