PITTSBURGH — The Pittsburgh Pirates starting rotation has had a strong showing so far in May, but they'll have their biggest test yet this weekend. The Pirates will host their in-state rivals in the Philadelphia Phillies at PNC Park, May 15-17, as a part of rivalry week, where teams across baseball face off against their most disliked ballclub. Pittsburgh's starting pitchers have given up 21 earned runs over 62.1 innings for a 3.03 ERA this month and are finally seeing the success they hoped for when they started the season. The Pirates aren't the only team with a great rotation, as the Phillies are bringing in some quality arms as well for this series. Cy Young Battle in the Series Finale The most anticipated starting pitcher matchup this series will come in the series finale on May 17, as Pirates right-hander Paul Skenes takes the mound against Phillies right-hander Zack Wheeler.…