typing: fix QPolygon << operator

Reported from PySide6-stubs

QPolygon operator << has incorrect return type.

The following should be correctly type-checked but is not:

point = QPoint()
point_list = [point]
polygon = QPolygon()

polygon << point << point
polygon << [point, point] << [point, point] << (point, point) << (point, point)

Current signature is:

    @typing.overload
    def _lshift(self, arg_1: PySide6.QtCore.QPoint, /) -> None: ...
    @typing.overload
    def _lshift(self, arg_1: typing.Sequence[PySide6.QtCore.QPoint], /) -> None: ...

It works correctly if we change it to:

    @overload
    def _lshift(self, arg_1: PySide6.QtCore.QPoint) -> PySide6.QtGui.QPolygon: ...
    @overload
    def _lshift(self, arg_1: Sequence[PySide6.QtCore.QPoint]) -> PySide6.QtGui.QPolygon: ...

Note that the same fix should be done to QPolygonF