A.R. Gurney’s play “Sylvia,” which hits the stage this weekend and next at the Hoogland Center for the Arts, is about a love triangle involving Greg and Kate, who are married, and Sylvia — a dog, whose role in performed by a human. Greg, who found Sylvia in the park, wants to keep her around. Kate, who is looking forward to the couple’s empty-nest years, does not want the dog around, especially when she starts getting in the way of Greg’s job and their marriage.