Add self highlight

syntax/python.vim

 syn keyword pythonImport        import
 syn keyword pythonException     try except finally
 syn keyword pythonOperator      and in is not or
+syn keyword pythonSelf          self
 
 syn match pythonStatement   "\<yield\>" display
 syn match pythonImport      "\<from\>" display
   HiLink pythonRepeat           Repeat
   HiLink pythonException        Exception
   HiLink pythonOperator         Operator
+  HiLink pythonSelf             Special
 
   HiLink pythonDecorator        Define
   HiLink pythonDottedName       Function