convert xlwt Workbook instance to a BytesIO instance

def xlsx_to_strio(xlsx_wb):
    """
        convert xlwt Workbook instance to a BytesIO instance
    """
    _xlrd_required()
    fh = BytesIO()
    xlsx_wb.filename = fh
    xlsx_wb.close()
    # prep for reading
    fh.seek(0)
    return fh