Stop using sys.maxint

It doesn't exist in python3, but 1000 days should be safe enough as a
fallback for a package without urgency.

Signed-off-by: Julien Cristau <jcristau@debian.org>

britney.py

@@ -789,7 +789,7 @@ class Britney(object):
 
             # read the minimum days associated with the urgencies
             urgency_old = urgencies.get(l[0], None)
-            mindays_old = self.MINDAYS.get(urgency_old, sys.maxint)
+            mindays_old = self.MINDAYS.get(urgency_old, 1000)
             mindays_new = self.MINDAYS.get(l[2], self.MINDAYS[self.options.default_urgency])
 
             # if the new urgency is lower (so the min days are higher), do nothing