python: Fix Database().needs_upgrade()

A stupid typo was preventing this from ever working and it was not
detected until now. Patrick noted the typo and proposed the fix in mail
id:"20110704203926.GA20238@brick.lan".

Patch-by: Patrick Totzke <patricktotzke@googlemail.com>
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>