gh-126209: Fix inconsistency of `skip_file_prefixes` in `warnings.warn`'s C and Python implementations (GH-126329)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Kirill Podoprigora <kirill.bast9@mail.ru>