Add `aclosing()` around asyn gen loop

examples/parallelism/concurrent_actors_primes.py

@@ -16,6 +16,7 @@ import time
 
 import tractor
 import trio
+from async_generator import aclosing
 
 
 PRIMES = [
@@ -103,8 +104,9 @@ async def main():
     async with worker_pool() as actor_map:
 
         start = time.time()
-        # for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
-        async for number, prime in actor_map(is_prime, PRIMES):
+
+        async with aclosing(actor_map(is_prime, PRIMES)) as results:
+            async for number, prime in results:
 
                 print(f'{number} is prime: {prime}')