amaury> since the prompt is written to stderr, why is sys.stdout.encoding
amaury> used instead of sys.stderr.encoding?

input() calls PyOS_Readline() but PyOS_Readline() has multiple 
implementations:
 - PyOS_StdioReadline() if sys_stdin or sys_stdout is not a TTY
 - or PyOS_ReadlineFunctionPointer callback:
   - vms__StdioReadline() (VMS only)
   - PyOS_StdioReadline()
   - call_readline() when readline module is loaded

call_readline() calls rl_callback_handler_install() with the prompt which 
writes the prompt to *stdout* (try ./python 2>/dev/null).

I don't think that it really matters that the prompt is written to stderr with 
stdout encoding, because both outputs always use the same encoding.