`GPollableInputStream` is implemented by [class@Gio.
InputStream]s that can be polled for readiness to read. 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 `GPollableInputStream` but have only certain instances of that class be pollable. If [
method@Gio.PollableInputStream.can_poll] returns false, then the behavior of other `GPollableInputStream` methods is undefined.