CPython's implementation has quite a bit more features than our
old one.  We only particularly need one of them (call __init__
when accessed from a new thread), but it looks like there are
some other features in there that have a decent chance of biting
us in annoying ways (some gc-related stuff).

That implementation forced some of the other work in this PR, of
supporting weakrefs on extension objects (which this uses), and
making object.tp_init get set the same way it does in CPython.