`GPollableOutputStream` is implemented by [class@Gio.
OutputStream]s that can be polled for readiness to write. This can be used when interfacing with a non-GIO API that expects
UNIX-file-descriptor-style asynchronous I/O rather than GIO-style.
Some classes may implement `GPollableOutputStream` but have only certain instances of that class be pollable. If [
method@Gio.PollableOutputStream.can_poll] returns false, then the behavior of other `GPollableOutputStream` methods is undefined.