Serialize our changes back to the PDF in memory

        Depending how we are initialized, leave our metadata mark and producer.

def _apply_changes(self):
        """Serialize our changes back to the PDF in memory

        Depending how we are initialized, leave our metadata mark and producer.
        """
        if self.mark:
            self[QName(XMP_NS_XMP, 'MetadataDate')] = datetime.now().isoformat()
            self[QName(XMP_NS_PDF, 'Producer')] = 'pikepdf ' + pikepdf_version
        xml = self._get_xml_bytes()
        self._pdf.Root.Metadata = Stream(self._pdf, xml)
        self._pdf.Root.Metadata[Name.Type] = Name.Metadata
        self._pdf.Root.Metadata[Name.Subtype] = Name.XML
        if self.sync_docinfo:
            self._update_docinfo()