> -u doesn't affect stdin buffering, whatever that would mean.

I think we need to document behavior of stdin somewhere, because current the sys.stdin documentation states:

> When interactive, standard streams are line-buffered. Otherwise, they
> are block-buffered like regular text files. You can override this value
> with the -u command-line option.

https://docs.python.org/3/library/sys.html#sys.stdin