Replace deprecation warning with RuntimeError (GH-14397)

Author: asvetlov

Lib/asyncio/streams.py

@@ -1293,10 +1293,8 @@ def __init__(self, mode, *,
                  is_server_side=False,
                  _asyncio_internal=False):
         if not _asyncio_internal:
-            warnings.warn(f"{self.__class__} should be instaniated "
-                          "by asyncio internals only, "
-                          "please avoid its creation from user code",
-                          DeprecationWarning)
+            raise RuntimeError(f"{self.__class__} should be instantiated "
+                               "by asyncio internals only")
         self._mode = mode
         self._transport = transport
         self._protocol = protocol

Lib/test/test_asyncio/test_streams.py

@@ -1779,6 +1779,12 @@ async def test():
 
         self.loop.run_until_complete(test())
 
+    def test_stream_ctor_forbidden(self):
+        with self.assertRaisesRegex(RuntimeError,
+                                    "should be instantiated "
+                                    "by asyncio internals only"):
+            asyncio.Stream(asyncio.StreamMode.READWRITE)
+
 
 if __name__ == '__main__':
     unittest.main()