I think I understand what's going on.  The trail leads from the last "if
(tty) {" block in builtin_input() to PyOS_Readline() which in turn ends
up calling PyOS_StdioReadline() (because that's the most likely
initialization of PyOS_ReadlineFunctionPointer).  And this, finally,
uses fprintf() to stderr to print the prompt.  That apparently doesn't
use the same encoding, or perhaps by now the string has been encoded as
UTF-8.

This is clearly a problem.  But what to do about it...