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

[serhiy-storchaka]

• 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 e2f7107)

• Issue: Take into account encoding of source file for syntax error #124188