Send stderr of ping tracker to devnull (#7096)

When pinging an inaccessible device, OS errors like

    ping: sendto: No route to host

can occur. For a ping tracker this is not an error but rather a normal
situation. Thus, it makes sense to hide the error.

homeassistant/components/device_tracker/ping.py

@@ -45,7 +45,9 @@ class Host(object):
 
     def ping(self):
         """Send an ICMP echo request and return True if success."""
-        pinger = subprocess.Popen(self._ping_cmd, stdout=subprocess.PIPE)
+        pinger = subprocess.Popen(self._ping_cmd,
+                                  stdout=subprocess.PIPE,
+                                  stderr=subprocess.DEVNULL)
         try:
             pinger.communicate()
             return pinger.returncode == 0