use a stack of self._fds_to_close to prevent double closes

[graingert]

and make tests easier to write because the close order is deterministic and in the order that opens happen in

this should also be a bit faster because list.append is faster than set.add and we skip a call to os_close(-1) and catching an OSError exception

[graingert]

@fantix can I get a review on this please?

[graingert]

@fantix thanks for the review!