Add missing import

Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/unit.py

@@ -27,6 +27,7 @@ from django.contrib import messages
 from django.core.cache import cache
 import traceback
 import multiprocessing
+from functools import reduce
 from weblate.trans.checks import CHECKS
 from weblate.trans.models.source import Source
 from weblate.trans.models.unitdata import Check, Comment, Suggestion