examples: avoid "x == None" checks in python examples

lgmt.com says "Testing for None should use the is operator".

examples/python/dbus/checkpoint.py

@@ -41,7 +41,7 @@ devList = []
 
 for arg in sys.argv[2:]:
     path = GetDevicePath(arg)
-    if path == None:
+    if path is None:
         raise Exception("NetworkManager knows nothing about %s" % arg)
     else:
         devList.append(path)