Remove unneeded import; avoid duplicate checks in cleanup().

src/netns/node.py

 # vim:ts=4:sw=4:et:ai:sts=4
-import errno, os, socket, sys, traceback, unshare, weakref
+import os, socket, sys, traceback, unshare, weakref
 from netns.environ import *
 import netns.interface, netns.protocol, netns.subprocess_
 

src/netns/protocol.py

@@ -97,9 +97,9 @@ class Server(object):
                 # -PID to kill to whole process group
                 os.kill(-pid, signal.SIGTERM)
             now = time.time()
-            ch = self._children
+            ch = set(self._children)
             while time.time() - now < KILL_WAIT:
-                for pid in set(ch):
+                for pid in ch:
                     try:
                        if netns.subprocess_.poll(pid):
                            ch.remove(pid)