asyncio: mark positional-only args (#3934)

tests/stubtest_whitelists/py38.txt

@@ -9,14 +9,7 @@ asyncio.AbstractEventLoop.sock_sendfile
 asyncio.Future.__init__
 asyncio.Future._callbacks
 asyncio.Future._schedule_callbacks
-asyncio.Future.remove_done_callback
-asyncio.Future.set_exception
-asyncio.Future.set_result
 asyncio.Handle.__init__
-asyncio.Task.remove_done_callback
-asyncio.Task.set_exception
-asyncio.Task.set_name
-asyncio.Task.set_result
 asyncio.TimerHandle.__init__
 asyncio.events.AbstractEventLoop.sock_sendfile
 asyncio.events.Handle.__init__
@@ -24,14 +17,7 @@ asyncio.events.TimerHandle.__init__
 asyncio.futures.Future.__init__
 asyncio.futures.Future._callbacks
 asyncio.futures.Future._schedule_callbacks
-asyncio.futures.Future.remove_done_callback
-asyncio.futures.Future.set_exception
-asyncio.futures.Future.set_result
 asyncio.proactor_events._ProactorBasePipeTransport.__del__
-asyncio.tasks.Task.remove_done_callback
-asyncio.tasks.Task.set_exception
-asyncio.tasks.Task.set_name
-asyncio.tasks.Task.set_result
 bdb.Bdb.runcall
 builtins.bytearray.extend
 builtins.bytearray.pop