view changes for py3

Python3 changes several methods on dict's to views, requiring us to
convert them to lists.

Change-Id: Ib08c03564f198d1c08142c44bf9baac6a73816dd
diff --git a/zuul/manager/__init__.py b/zuul/manager/__init__.py
index bdfde48..5b32e5b 100644
--- a/zuul/manager/__init__.py
+++ b/zuul/manager/__init__.py
@@ -426,7 +426,7 @@
             build.result = 'CANCELED'
             canceled = True
             canceled_jobs.add(build.job.name)
-        for jobname, nodeset in old_build_set.nodesets.items()[:]:
+        for jobname, nodeset in list(old_build_set.nodesets.items()):
             if jobname in canceled_jobs:
                 continue
             self.sched.nodepool.returnNodeSet(nodeset)