asyncio: sync with Tulip

* Remove unused SSLProtocol._closing attribute
* test_sslproto: skip test if ssl module is missing
* Python issue #23208: Don't use the traceback of the current handle if we
  already know the traceback of the source. The handle may be more revelant,
  but having 3 tracebacks (handle, source, exception) becomes more difficult to
  read. The handle may be preferred later but it requires more work to make
  this choice.

Lib/asyncio/sslproto.py

@@ -408,7 +408,6 @@ class SSLProtocol(protocols.Protocol):
         self._write_buffer_size = 0
 
         self._waiter = waiter
-        self._closing = False
         self._loop = loop
         self._app_protocol = app_protocol
         self._app_transport = _SSLProtocolTransport(self._loop,