[3.13] gh-119521: Rename IncompleteInputError to _IncompleteInputError and remove from public API/ABI (GH-119680, GH-120955) (GH-120944)

- gh-119521: Rename IncompleteInputError to _IncompleteInputError and remove from public API/ABI (GH-119680)
  (cherry picked from commit ce1064e4c9bcfd673323ad690e60f86e1ab907bb)

- gh-119521: Use `PyAPI_DATA`, not `extern`, for `_PyExc_IncompleteInputError` (GH-120955)
  (cherry picked from commit ac61d58db0753a3b37de21dbc6e86b38f2a93f1b)

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