By Dave Graham MEXICO CITY (Reuters) – A U.S. decision to suspend avocado shipments from a violent Mexican state sealed a challenging week for President Andres Manuel Lopez Obrador, whose approval rating suffered an unusual dip in a tracking poll published on Sunday. Lopez Obrador, whose still robust popularity has been more resilient than most global leaders, has this month become embroiled in a war of words with a critical journalist over his reporting on the lifestyle of one of the president’s sons. On Friday, the president devoted a chunk of his daily news conference to criticizing journal…