typing: QObject.findChildren() and findChild() need typing improvements

Reported from PySide6-stubs

QObject.findChildren() and findChild() need typing improvements.

The following example should work:

    a: List[QObject]
    a = o1.findChildren(QObject)

But it fails on two reasons:

• findChildren() returns Iterable[] but this prevents assigning the result to an actual list. For input values, it is a good idea to be generic and use Iterable[] (covariant type). For return values, it is a good idea to be very specific (contravariant type)

• the type of the return value of findChildren() is not type-checked against the type we are looking for.

For example, the following code should generate a typing error, but it does not:

    c: List[QWidget]
    c = o1.findChildren(QObject, '')

See https://github.com/python-qt-tools/PySide6-stubs/blob/main/tests/qobject.py for a full example.

The correct signature to have propre type-checking is the following:

@overload
def findChildren(self, type: Type[PlaceHolderType], name: str = ..., options: PySide6.QtCore.Qt.FindChildOption = ...) -> List[PlaceHolderType]: ...
@overload
def findChildren(self, type: Type[PlaceHolderType], pattern: Union[PySide6.QtCore.QRegularExpression, str], options: PySide6.QtCore.Qt.FindChildOption = ...) -> List[PlaceHolderType]: ...

This applies also to findChild():

def findChild(self, type: Type[PlaceHolderType], name: str = ..., options: PySide6.QtCore.Qt.FindChildOption = ...) -> Optional[PlaceHolderType]: ...