OAKLAND, Calif. — The Athletics dealt the New York Mets a costly loss in a pennant race Saturday and did it against star right-handed pitcher Jacob deGrom. The Athletics gave deGrom a harder time than he’d seen in his previous nine starts and parlayed clutch hitting and solid defense for a 10-4 win at the at the Coliseum. The Athletics improved to 56-96 before a crowd of 16,041, many of them Mets fans. New York, which began the day with a 2 1/2-game lead over the Atlanta Braves in the National League East, fell to 96-57. By the time he was finished after four innings, deGrom (5-3) trailed 5-4 …