It seems like that some methods of the io.TextIOWrapper class requires that its buffer object has the read1() method, whereas the constructor checks if the buffer has a read1() method and the TextIOWrapper _read_chunk() method is able to call buffer.read() if buffer doesn't have read1().

This issue may help to get fully unbuffered sys.stdin, at least when replaced manually:

stdin = sys.stdin
sys.stdin = open(0, "r", buffering=0, encoding=stdin.encoding, errors=stdin.errors, newline=stdin.newline)