Okay! I think I have something here. DEFINITELY needs more eyeballs, but all tests pass, including a new one that tests StopIteration leakage both with and without the __future__ directive. Some docs changes have been made (I grepped for 'stopiteration' and 'generator' case insensitively, and changed anything that caught my eye). It'd be nice if the entire test suite and standard library could be guaranteed to work in a post-3.7 world, but I don't know of an easy way to do that, short of peppering the code with unnecessary __future__ directives.