bpo-40943: Fix skipitem() didn't raise SystemError (GH-25937)

`convertitem()` raises `SystemError` when 'GH-' is used without `PY_SSIZE_T_CLEAN`.
This commit makes `skipitem()` raise it too.
(cherry picked from commit 4ebf4a6bfad4afcbab3baf9c0159c7767e2a64c0)

Co-authored-by: Inada Naoki <songofacandy@gmail.com>