runtime(python): highlight "self" and "cls" in syntax script

These are special names by convention, and giving them distinct
highlighting is a nice visual clue (using Identifier by default).

This group is named "pythonClassVar" to match the name used by
python-syntax. Some third-party color schemes are aware of this
name and customized their colors accordingly.

closes: #17968

Signed-off-by: Jon Parise <[email protected]>
Signed-off-by: Zvezdan Petkovic <[email protected]>
Signed-off-by: Christian Brabandt <[email protected]>

Author: jparise

runtime/syntax/python.vim

@@ -1,7 +1,7 @@
 " Vim syntax file
 " Language:	Python
 " Maintainer:	Zvezdan Petkovic <[email protected]>
-" Last Change:	2025 Aug 11
+" Last Change:	2025 Aug 13
 " Credits:	Neil Schemenauer <[email protected]>
 "		Dmitry Vasiliev
 "		Rob B
@@ -113,6 +113,10 @@ syn keyword pythonAsync		async await
 syn match   pythonConditional   "^\s*\zscase\%(\s\+.*:.*$\)\@="
 syn match   pythonConditional   "^\s*\zsmatch\%(\s\+.*:\s*\%(#.*\)\=$\)\@="
 
+" These names are special by convention. While they aren't real keywords,
+" giving them distinct highlighting provides a nice visual cue.
+syn keyword pythonClassVar	self cls
+
 " Decorators
 " A dot must be allowed because of @MyClass.myfunc decorators.
 syn match   pythonDecorator	"@" display contained
@@ -378,6 +382,7 @@ hi def link pythonOperator		Operator
 hi def link pythonException		Exception
 hi def link pythonInclude		Include
 hi def link pythonAsync			Statement
+hi def link pythonClassVar		Identifier
 hi def link pythonDecorator		Define
 hi def link pythonDecoratorName		Function
 hi def link pythonClass			Structure