Fix missing import for issue724

invoice.py

@@ -3,8 +3,6 @@
 "Invoice"
 
 from trytond.osv import fields, OSV
-import datetime
-import mx.DateTime
 from decimal import Decimal
 from trytond.netsvc import LocalService
 from trytond.report import Report

payment_term.py

@@ -4,6 +4,8 @@
 from trytond.osv import fields, OSV
 from trytond.sql_db import table_handler
 from decimal import Decimal
+import datetime
+import mx.DateTime
 
 
 class PaymentTerm(OSV):