Return the previous sibling of this node.

        If there is no previous sibling, return ``None``.

def previousSibling(self) -> Optional[AbstractNode]:
        """Return the previous sibling of this node.

        If there is no previous sibling, return ``None``.
        """
        parent = self.parentNode
        if parent is None:
            return None
        return parent.childNodes.item(parent.childNodes.index(self) - 1)