MINNEAPOLIS — Relief, even if fleeting and momentary, is a feeling that Black Americans have rarely known in America: From slavery to Jim Crow segregation to enduring punishments for living while Black, a breath of fresh air untainted by oppression has long been hard to come by. Nonetheless, the conviction of ex-cop Derek Chauvin for murdering George Floyd nearly a year ago allowed many across this city and the nation to exhale pent-up anxiety — and to inhale a sense of hope. But what might they feel hope for? The fate of Chauvin — found guilty of murder and manslaughter for holding a knee to Floyd’s neck, choking off his breathing until he went limp last May — showed Black Americans and their compatriots once again that the legal system is capable of valuing Black lives.