lingo: add back date parser import as it's still in use (#24079)

combo/apps/lingo/models.py

@@ -23,6 +23,7 @@ import urlparse
 
 from decimal import Decimal
 
+from dateutil import parser
 import eopayment
 from jsonfield import JSONField
 

tests/test_lingo_remote_regie.py

@@ -36,7 +36,7 @@ INVOICES = [
         'has_pdf': True,
         'online_payment': True,
         'paid': False,
-        'payment_date': None,
+        'payment_date': '1970-01-01',
         'no_online_payment_reason': '',
     },
 ]