Use os._exit directly instead of raising SystemExit
There appears to be a cleanup race when raising SystemExit, so exit
directly.
@@ -54,7 +54,7 @@ async def worker(cls, cls_args, sockname):
try:
req = await con.next_request()
except amsg.PoolClosedError:
- raise SystemExit
+ os._exit(0)
try:
methname, args = pickle.loads(req)
