Plays by women; plays about women. The fairer sex captures the imagination of many D.C. theaters this September, offering works by celebrated female authors and performing plays that focus on female characters. And if that’s not your thing, well, there’s always Kafka. Two area theaters present works by Caryl Churchill — Studio Theatre performing A Number beginning Sept. 7, and Fountainhead Theatre staging Top Girls, which opens Sept. 8. The former show explores ethical issues…