Make future week check more pythonic

While writing this simple patch, I wasted an hour debugging a problem where
datetime would throw an error /only/ after the addon had been run once...

http://forum.xbmc.org/showthread.php?tid=112916

I don't know what the real problem is, but I think it's a Python bug.
Bumping xbmc.python to 2.14 (for Gotham) did not help. :-/