It’s impossible to discuss Pen Pals, Michael Griffo’s new two-hander at the Theatre at St. Clement’s, without first bringing up A.R. Gurney’s Love Letters. A Pulitzer Prize finalist, Love Letters consisted of nothing more than two actors reading letters to each other, recounting an epistolary romance spanning almost a half-century. It was so popular because, first of all, it was easy to produce: small set, small cast, and celebrity actors who could jet into town, get onstage, and read the text without having to memorize anything.