One of 2020’s few joys has been Steve McQueen’s Amazon anthology “Small Axe,” a series telling, in some instances for the first time, the stories of the Black Brits who faced oppression during the ’60s and ’70s. While “Mangrove” touted empowerment through self-representation, “Lovers Rock” through music, and “Red, White and Blue” through reform from within, “Alex Wheatle” calls for literature as a gateway to freedom. The film also features, among an uneven story and striking visuals, a skilled debut from Sheyi Cole, whose most powerful acting happens during the film’s quietest moments, as the title character.