britney: Use chain rather then duplicate statement

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

britney.py

@@ -186,6 +186,7 @@ import sys
 import time
 from collections import defaultdict
 from functools import reduce
+from itertools import chain
 from operator import attrgetter
 from urllib.parse import quote
 
@@ -2732,8 +2733,7 @@ class Britney(object):
 
                 for item, ver in items:
                     # excuses which depend on "item" or are depended on by it
-                    new_items = set((x, excuses[x].ver[1]) for x in excuses_deps[item])
-                    new_items.update((x, excuses[x].ver[1]) for x in excuses_rdeps[item])
+                    new_items = {(x, excuses[x].ver[1]) for x in chain(excuses_deps[item], excuses_rdeps[item])}
                     new_items -= seen_items
                     items.extend(new_items)
                     seen_items.update(new_items)