[3.12] gh-122845: fix parameter_list_starargs in function definition pseudo-grammar (GH-122847) (#129151)

gh-122845: fix parameter_list_starargs in function definition pseudo-grammar (GH-122847)

Thanks to Artur Chakhvadze for bugfix.
(cherry picked from commit 610584639003317193cdcfe38bbdc8268b612828)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>