Air carrier First Officer reported a traffic conflict with another aircraft while on the arrival. The flight crew followed the TCAS commands to descend and avoided the conflict.

Narrative

On BOJID4 RNAV Arrival; at HIFAL waypoint ATC advise us of Aircraft Y traffic in our vicinity to our 10 o'clock position conducting traffic pattern work at PNE. I responded searching for traffic; however said traffic was heard reporting us in sight with ATC relaying same information to us. At approximately EYRIE waypoint at 6;000ft we received a Traffic Advisory alert via TCAS. I immediately searched and found the traffic in our direction; increasing in image size; estimated 200-300 ft above us. Simultaneously I told the pilot flying (PF) to descend with the Resolution Advisory instructing us to descent. PF conducted RA as instructed. The aircraft eventually paralleled (same direction) us to our 8 o'clock position close enough to see some finer details of the aircraft. During the descent I informed ATC we were receiving RA instructions to which they asked us to let them know when we could descend; in which we responded we were descending at that time. ATC instructed us to descend to 5;000ft. Clear of conflict" was given before reaching 5;000ft.Cause: Traffic's loss of positional awareness as well as lesser than effective airspace traffic mitigation techniques.Suggestion: Traffic deconfliction measures need to be implemented at PNE such as restriction of type of traffic pattern (left/right pattern only) as well as altitude limitations due to arrivals at Class B PHL."