Add @pablogsal to code owners file for Parser/pgen (GH-12944)

Add myself to the codeowners file as I would like to
be automatically added as a reviewer for PRs that touch
that component.

.github/CODEOWNERS

@@ -43,6 +43,9 @@ Python/bootstrap_hash.c       @python/crypto-team
 **/*imap*                     @python/email-team
 **/*poplib*                   @python/email-team
 
+# Parser/Pgen
+/Parser/pgen/                 @pablogsal
+
 # SQLite 3
 **/*sqlite*                   @berkerpeksag