By closing them when we're done, we can remove reliance on GC to clean
up multiplexed watchers and solve problems for PyPy and Windows (and
probably eventually simplify the handle cleanup we were doing.)

This makes many more tests on PyPy pass on Darwin; down from a score
of failures to about 4, but a baffling 4. test__os:TestOS_nb times
out; investigation shows that both the read and write sides of the
pipe are waiting on events, both are in libuv, and the write side
should be writable, but it's not getting an event. Not clear why this
is. I also see it with CPython 3.6, and I don't think I did before.