By Denny Dyroff, Entertainment Editor, The Times  This year, the Sellersville Theater (24 West Temple Avenue, Sellersville, 215-257-5808, www.st94.com) has gone through a saga that is like that of the “Black Knight.” The Black Knight is a fictional character who appears in a scene of the feature film, “Monty Python and the Holy Grail.” In case your memory of him is blurry – or if you’ve...