“So defendants like Walter McMillian, even in counties that were 40 or 50 percent black, frequently found themselves staring at all-white juries, especially in death penalty cases. Then, in 1986, the Supreme Court ruled in Batson v. Kentucky that prosecutors could be challenged more directly about using peremptory strikes in a racially discriminatory manner, giving hope to black defendants – and forcing prosecutors to find more creative ways to exclude black jurors.”