The city of Chicago may not deny that police for years routinely hid exculpatory evidence from defendants battling criminal charges, a federal judge held Monday.In a written opinion, U.S. District Judge Joan H. Lefkow blocked the city from arguing in the trial of a civil rights lawsuit that it did not have a policy in the late 1980s of hiding evidence contained in so-called “street files” maintained by police.The suit was filed by James Kluppelberg, who spent 24 years in prison for an arson that killed six …