I would like to be able to retrieve some information about the signature of a SignalInstance (i.e. the parameters accepted by .emit) from only the SignalInstance itself (that is, without having access to the metaObject on the owner of the signal instance).

Take this example:

from PySide2.QtCore import Signal, QObject
import inspect

class MyObj(QObject):
    some_signal = Signal()

obj = MyObj()

Using inspect.signature gives erroneous info:

>>> inspect.signature(obj.some_signal.emit)
<Signature (*args: typing.Any) -> None>

>>> obj.some_signal.emit(1)  # compatible with the signature above
TypeError: some_signal() only accepts 0 argument(s), 1 given!

The signature is stored in the `PySideSignalInstancePrivate` struct, and would be nice to access.

As a sidenote: PyQt adds a .signal .signatures attribute (holding the string signature) to the bound signal instance, that is sufficient for retrieving the signature. Though returning a proper `Signature` instance from `SignalInstance.__signature__` feels more correct