By THÉRÈSE MARGOLIS     Every time the Big Three international rating agencies downgrade the country or it’s state-run oil venture, Petróleos Mexicanos (Pemex), Mexican President Andrés Manuel López Obrador (AMLO) proudly points to the fact that the peso is doing well against the U.S. dollar, with a parity now below 19 to 1. And in most financial universes, a strong currency