build: Fix compatibility with Python 3

Change-Id: I7a1b98c38c2ed3a147c7b0e6c7b79d2723f2a3ba
Refs: #3493
diff --git a/tests/wscript b/tests/wscript
index 3b9e628..3d080aa 100644
--- a/tests/wscript
+++ b/tests/wscript
@@ -42,7 +42,7 @@
 
         for module, name in {"core": "NFD Core Tests",
                              "daemon": "NFD Daemon Tests",
-                             "rib": "NFD RIB Tests"}.iteritems():
+                             "rib": "NFD RIB Tests"}.items():
             # main()
             bld(target='unit-tests-%s-main' % module,
                 name='unit-tests-%s-main' % module,