Mark None as Python builtin object along with True and False

More specifically, they are "built-in constants":

  https://docs.python.org/2/library/constants.html#built-in-consts

syntax/python.vim

   syn keyword pythonImport      as
   syn match   pythonFunction    "[a-zA-Z_][a-zA-Z0-9_]*" display contained
 else
-  syn keyword pythonStatement   as nonlocal None
+  syn keyword pythonStatement   as nonlocal
   syn match   pythonStatement   "\<yield\s\+from\>" display
-  syn keyword pythonBoolean     True False
+  syn keyword pythonBuiltinObj  None True False
   syn match   pythonFunction    "\%([^[:cntrl:][:space:][:punct:][:digit:]]\|_\)\%([^[:cntrl:][:punct:][:space:]]\|_\)*" display contained
   syn keyword pythonStatement   await
   syn match   pythonStatement   "\<async\s\+def\>" nextgroup=pythonFunction skipwhite
 
 if s:Enabled("g:python_highlight_builtin_objs")
   if s:Python2Syntax()
-    syn keyword pythonBuiltinObj	None
-    syn keyword pythonBoolean		True False
+    syn keyword pythonBuiltinObj	None False True
   endif
   syn keyword pythonBuiltinObj	Ellipsis NotImplemented
   syn keyword pythonBuiltinObj	__debug__ __doc__ __file__ __name__ __package__