The Signal class provides easy creation of signals and removes the
need for __gsignals__ in client code. The Signal class can also be
used as a decorator for wrapping up the custom closure. As well as
providing a "BoundSignal" when accessed on an instance for making
connections without specifying a signal name string.
Python3 annotations can also be used to supply closure argument and
return types when Signal is used as a decorator. For example:

class Eggs(GObject.GObject):
    @GObject.Signal
    def spam(self, count:int):
        pass

https://bugzilla.gnome.org/show_bug.cgi?id=434924