[3.9] Add lysnikolaou to CODEOWNERS for the new parser (GH-21253)

diff --git a/.github/CODEOWNERS b/.github/CODEOWNERS
index 4d80698..eb6fc2c 100644
--- a/.github/CODEOWNERS
+++ b/.github/CODEOWNERS
@@ -74,8 +74,9 @@
 
 # Parser/Pgen
 /Parser/pgen/                 @pablogsal
-/Parser/pegen/                @pablogsal
-/Tools/peg_generator/         @pablogsal
+/Parser/pegen/                @pablogsal @lysnikolaou
+/Tools/peg_generator/         @pablogsal @lysnikolaou
+/Lib/test/test_peg_generator/ @pablogsal @lysnikolaou
 
 # SQLite 3
 **/*sqlite*                   @berkerpeksag