Return a new |EmbeddedXlsxPart| instance added to *package* and
        containing *xlsx_blob*.

def new(cls, xlsx_blob, package):
        """
        Return a new |EmbeddedXlsxPart| instance added to *package* and
        containing *xlsx_blob*.
        """
        partname = package.next_partname(cls.partname_template)
        content_type = CT.SML_SHEET
        xlsx_part = cls(partname, content_type, xlsx_blob, package)
        return xlsx_part