By Elvira Pollina MILAN, Dec 19 (Reuters) - Italy's Treasury has agreed to sell PagoPA, which handles digital payments to public ​administrations, to the state mint and to state-backed postal ‌group Poste Italiane for up to 500 million euros ($586 million), the mint said ‌on Friday. The deal has raised concerns among Italian banks in recent months, as it could boost competition for smaller lenders already struggling to keep pace with rapid changes in the payments sector. The 500-million-euro valuation ...