asyncio: add asyncio.threads for py39 (#4136)

tests/stubtest_whitelists/py35.txt

@@ -4,7 +4,7 @@ asyncio.Future._loop
 asyncio.Future._tb_logger
 asyncio.Task.__init__
 asyncio.Task._wakeup
-asyncio.exceptions
+asyncio.exceptions  # Added in Python 3.8
 asyncio.futures.Future._callbacks
 asyncio.futures.Future._exception
 asyncio.futures.Future._loop
@@ -13,6 +13,7 @@ asyncio.futures._TracebackLogger.__init__
 asyncio.runners
 asyncio.tasks.Task.__init__
 asyncio.tasks.Task._wakeup
+asyncio.threads  # Added in Python 3.9
 bdb.GENERATOR_AND_COROUTINE_FLAGS
 builtins.str.maketrans
 cmath.log

tests/stubtest_whitelists/py36.txt

@@ -1,8 +1,9 @@
 asyncio.Future.__init__
-asyncio.exceptions
+asyncio.exceptions  # Added in Python 3.8
 asyncio.futures.Future.__init__
 asyncio.futures._TracebackLogger.__init__
 asyncio.runners
+asyncio.threads  # Added in Python 3.9
 builtins.str.maketrans
 cmath.log
 codecs.StreamRecoder.seek

tests/stubtest_whitelists/py37.txt

@@ -6,9 +6,10 @@ asyncio.TimerHandle.__init__
 asyncio.events.AbstractEventLoop.sock_sendfile
 asyncio.events.Handle.__init__
 asyncio.events.TimerHandle.__init__
-asyncio.exceptions
+asyncio.exceptions  # Added in Python 3.8
 asyncio.futures.Future.__init__
 asyncio.futures.Future._callbacks
+asyncio.threads  # Added in Python 3.9
 builtins.dict.get
 builtins.reversed
 builtins.str.maketrans

tests/stubtest_whitelists/py38.txt

@@ -16,6 +16,7 @@ asyncio.events.TimerHandle.__init__
 asyncio.futures.Future.__init__
 asyncio.futures.Future._callbacks
 asyncio.proactor_events._ProactorBasePipeTransport.__del__
+asyncio.threads  # Added in Python 3.9
 builtins.compile
 builtins.dict.get
 builtins.reversed