You might not want to watch “Taxi Driver” (1976) this week. But “Taxi Driver,” directed by Martin Scorsese and written by Paul Schrader, is one of the best portraits of a uniquely American kind of alienation: a spiritual isolation that too often explodes into violence. Through this act of violence, Travis believes he will finally be seen. But it might be “Taxi Driver’s” final twist that offers its gravest warning for our times. “Taxi Driver” is streaming on the Criterion Channel and Kanopy.