[3.12] gh-124188: Fix PyErr_ProgramTextObject() (GH-124189) (GH-124426)

* Detect source file encoding.
* Use the "replace" error handler even for UTF-8 (default) encoding.
* Remove the BOM.
* Fix detection of too long lines if they contain NUL.
* Return the head rather than the tail for truncated long lines.
(cherry picked from commit e2f710792b0418b8ca1ca3b8cdf39588c7268495)