pingnet: Fix ResourceWarning

pingnet.py

@@ -64,9 +64,10 @@ class Pinger(threading.Thread):
                 break
 
     def ping(self, host):
+        with open(os.devnull, 'r+') as nul:
             p = subprocess.Popen(['ping', '-c', '1', '-W', '1', host],
-                             stdin=open(os.devnull),
-                             stdout=open(os.devnull, 'w'),
+                                 stdin=nul,
+                                 stdout=nul,
                                  stderr=subprocess.STDOUT)
         p.wait()
         if p.returncode == 0: