I have left some comments on Rietveld after a partial code review.

IMO for reasons of repeatability and easier maintenance [1], all the tests should be run with the pty set in a known mode, whether canonical or raw, and a test involving pty.spawn() should ensure that the chosen mode will be set by this function, or should skip it otherwise.

[1] For example when investigating the random failures in PtyTest reported by Cornelius in the patch comments, one has to wonder: "Oh, maybe it fails on this foreign system because the pty is not in raw mode there".