More than 40 years ago, this Court concluded that “exceptional conditions” prevailing in certain parts of the country justified extraordinary legislation otherwise unfamiliar to our federal system. In part due to the success of that legislation, we are now a very different Nation. Whether conditions continue to justify such legislation is a difficult constitutional question we do not answer today. We conclude instead that the Voting Rights Act permits all political subdivisions, including the district in this case, to seek relief from its preclearance requirements