[3.10] bpo-45822: Respect PEP 263's coding cookies in the parser even if flags are not provided (GH-29582) (GH-29586)

(cherry picked from commit da20d7401de97b425897d3069f71f77b039eb16f)

Co-authored-by: Pablo Galindo Salgado <Pablogsal@gmail.com>