IN MEXICO’S LEFTIST populist government, Carlos Urzúa, the social-democratic finance minister, was a reassuring figure. The president, Andrés Manuel López Obrador, has unorthodox ideas about how to develop Mexico. Mr Urzúa (pictured, right) would help make sure, investors hoped, that he pursued them without wrecking the economy. But on July 9th, after seven months in…