Add pythonConvention HL class (`cls` and `self`)

These are especially useful in OOP as they help visually distinguish how
many parameters the function will be called with (in signature), or
actual state preservation from transient or return variables (in
function body).

syntax/python.vim

   syn keyword pythonExClass	ImportWarning UnicodeWarning
 endif
 
+syn keyword pythonConvention    self
+syn keyword pythonConvention    cls
+
+
 if s:Enabled("g:python_slow_sync")
   syn sync minlines=2000
 else
 
   HiLink pythonExClass          Structure
 
+  HiLink pythonConvention       Define
+
   delcommand HiLink
 endif