gh-114914: Avoid keeping dead StreamWriter alive (#115661)

In some cases we might cause a StreamWriter to stay alive even when the
application has dropped all references to it. This prevents us from
doing automatical cleanup, and complaining that the StreamWriter wasn't
properly closed.

Fortunately, the extra reference was never actually used for anything so
we can just drop it.

Lib/test/test_asyncio/test_streams.py

@@ -1130,6 +1130,31 @@ os.close(fd)
 
         self.assertEqual(messages, [])
 
+    def test_unclosed_server_resource_warnings(self):
+        async def inner(rd, wr):
+            fut.set_result(True)
+            with self.assertWarns(ResourceWarning) as cm:
+                del wr
+                gc.collect()
+                self.assertEqual(len(cm.warnings), 1)
+                self.assertTrue(str(cm.warnings[0].message).startswith("unclosed <StreamWriter"))
+
+        async def outer():
+            srv = await asyncio.start_server(inner, socket_helper.HOSTv4, 0)
+            async with srv:
+                addr = srv.sockets[0].getsockname()
+                with socket.create_connection(addr):
+                    # Give the loop some time to notice the connection
+                    await fut
+
+        messages = []
+        self.loop.set_exception_handler(lambda loop, ctx: messages.append(ctx))
+
+        fut = self.loop.create_future()
+        self.loop.run_until_complete(outer())
+
+        self.assertEqual(messages, [])
+
     def _basetest_unhandled_exceptions(self, handle_echo):
         port = socket_helper.find_unused_port()