[FIX] account_financial_report: open_items

* Missing import causes failure when running Open Items with Date At in the past

account_financial_report/report/open_items.py

@@ -2,6 +2,7 @@
 # Copyright 2020 ForgeFlow S.L. (https://www.forgeflow.com)
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
+import operator
 from datetime import date, datetime
 
 from odoo import api, models