Set xlsx column widths using fld2col_widths.

def set_xlsx_colwidths(worksheet, fld2col_widths, fldnames):
        """Set xlsx column widths using fld2col_widths."""
        for col_idx, fld in enumerate(fldnames):
            col_width = fld2col_widths.get(fld, None)
            if col_width is not None:
                worksheet.set_column(col_idx, col_idx, col_width)