ui: do not use rawinput() when we have to replace sys.stdin/stdout
See the inline comment for why. The current Python3 hack doesn't work if
more than one user inputs are expected because TextIOWrapper fills its
internal buffer at the first read() request.
Maybe we could write an unbuffered TextIOWrapper, but I don't want to make
things more complicated. Instead, this patch reinvents raw_input(' ') of
no readline support.