According to the narrative, "even Negroes" can vote in Yoknapatawpha elections "now" - though these enfranchised voters "vote for the same . . . white supremacy champions that the white" voters elected (38).
The novel identifies the voters in Yoknapatawpha during the hundred years after the building of the courthouse in 1835 as "the white male citizens of the county" (37). Historically of course, a pair of Constitutional amendments gave black males the right to vote in 1870 and women the vote in 1920, but perhaps the novel is reminding us about the peculiar (unwritten) constitution of the world Faulkner is representing.