I think the difference between Python 2 and 3 here is that Python 2’s file objects, including sys.stdin, wrap C library FILE objects, which is supported by the Readline library. However Python 3 has its own kind of file objects, independent of standard C and Readline. Python 3 only uses Readline if sys.stdin corresponds to the original C stdin FILE object.

Perhaps Python 3 could support Readline with other file objects (or at least file descriptors), but I think that would be a new feature.