Proper warning level of deprecation notice

This enables us to control emitted messages via the PYTHONWARNINGS
environment variable or by -W option.

Author: mymedia2

httpx/_models.py

@@ -876,15 +876,17 @@ def __repr__(self) -> str:
     def stream(self):  # type: ignore
         warnings.warn(  # pragma: nocover
             "Response.stream() is due to be deprecated. "
-            "Use Response.aiter_bytes() instead."
+            "Use Response.aiter_bytes() instead.",
+            DeprecationWarning,
         )
         return self.aiter_bytes  # pragma: nocover
 
     @property
     def raw(self):  # type: ignore
         warnings.warn(  # pragma: nocover
             "Response.raw() is due to be deprecated. "
-            "Use Response.aiter_raw() instead."
+            "Use Response.aiter_raw() instead.",
+            DeprecationWarning,
         )
         return self.aiter_raw  # pragma: nocover