* `~.PopenSpawn` now has a ``preexec_fn`` parameter, like :class:`~.spawn`
and :class:`subprocess.Popen`, for a function to be called in the child
process before executing the new command. Like in ``Popen``, this works only
in POSIX, and can cause issues if your application also uses threads.
* Significant performance improvements when processing large amounts of data.
* Ensure that ``spawn.closed`` gets set by `~.spawn.close`, and improve
an example for passing ``SIGWINCH`` through to a child process.