glib::wrapper! {
    /// The `Purrable` interface provides a virtual method `is_purring`
    pub struct Purrable(ObjectInterface<iface::Purrable>);
}

This implements IsA<Object> etc, which is wrong for the interface implemented in https://github.com/gtk-rs/gtk-rs-core/blob/9f592fd3a85ea6889637586fb395819e06188c33/examples/virtual_methods/purrable.rs

We should probably automatically make GObject a prerequisite if none are given at all (i.e. not () but not at all) via #[glib::object_interface] as this is what most code will actually want, but the wrapper macro also needs fixes of some sort.

CC @felinira