“I used to be a writer,” Fran Lebowitz mentions in passing, in the first scene of the first episode of her new Netflix docu-series “Pretend It’s a City,” and she gives this information the weight she feels it deserves – which is to say, very little. That’s her nature; she was, in fact, quite an important writer, and her “Metropolitan Life” and “Social Studies” are among the finest and funniest chronicles of New York life ever published. But the latter was released in 1981, and she has now spent considerably more time suffering from writer’s block than she spent, y’know, writing.