fix: fix deletelist.sort for python 3

fixes:

Traceback (most recent call last):
  File "./makesnapshots.py", line 197, in <module>
    deletelist.sort(date_compare)

makesnapshots.py

@@ -187,14 +187,10 @@ def set_resource_tags(resource, tags):
             logging.info(snap)
             logging.info(snap.start_time)
 
-        def date_compare(snap1, snap2):
-            if snap1.start_time < snap2.start_time:
-                return -1
-            elif snap1.start_time == snap2.start_time:
-                return 0
-            return 1
-
-        deletelist.sort(date_compare)
+        def date_key(snap):
+            return snap.start_time
+
+        deletelist.sort(key=date_key)
         if period == 'day':
             keep = keep_day
         elif period == 'week':