About issue9198.diff:
 - exit directly if !PyErr_ExceptionMatches(PyExc_UnicodeEncodeError) to avoid an useless level of indentation
 - why do you clear the exception before calling PyObject_Repr()? if you cannot execute code while an exception is active, you should maybe save/restore the original exception?
 - the code is long: it can maybe be moved to a subfunction
 - PyObject_CallMethod(buffer, "write", "(O)", encoded) to call write method
 - you can maybe just use strict error handler when you decode the encoded variable: it doesn't work anyway


>>> b"a\xff".decode("ascii", "backslashreplace")
...
TypeError: don't know how to handle UnicodeDecodeError in error callback
>>> b"a\xff".decode("utf-8", "backslashreplace")
...
TypeError: don't know how to handle UnicodeDecodeError in error callback

Note: some encodings don't support backslashreplace, especially mbcs encoding. But on Windows, sys.stdout.encoding is not mbcs but cpXXXX (eg. cp850).