Import reduce via functools

In Python3 reduce must be imported from functools, python2.6
supports the import as a "nop".

Signed-off-by: Niels Thykier <niels@thykier.net>

britney.py

@@ -194,6 +194,7 @@ from excuse import Excuse
 from migrationitem import MigrationItem, HintItem
 from hints import HintCollection
 from britney import buildSystem
+from functools import reduce
 
 __author__ = 'Fabio Tranchitella and the Debian Release Team'
 __version__ = '2.0'