MEXICO CITY — China has finally invited Mexico to the party and is letting it bring its finest booze, ending a de facto ban on pure agave tequila.   Tequila makers are celebrating the long-awaited opening of China’s market after President Xi Jinping came here in May and, among other things, announced China would relax the restrictions that have prevented brands like Herradura, Patron, Don Julio, and José Cuervo from exporting there. The industry envisions positioning its tequila ...