Initial implementation checked in at https://hg.python.org/cpython/rev/e729b946cc03

Larry, FYI regarding the refleak in test_importlib I just committed: as described in the commit message, I'm pretty sure it's a real refleak in the current PEP 489 implementation, it's just a beast to track down because there are a lot of moving parts when it comes to module cleanup, especially for extension modules.

Aside from that, the tests all pass, so I hope you're OK with my checking it in its current state for beta 1, with the aim of fixing the bug for beta 2 (I would have asked first, but time zones intervened - I'm about to head to bed, and I expect you'll have closed 3.5 to new features by the time I get up)