Update _models.py (#2840)

To remove the unknown dict type info

(variable) extensions: ResponseExtensions | dict[Unknown, Unknown]

Co-authored-by: Tom Christie <tom@tomchristie.com>

httpx/_models.py

@@ -467,7 +467,7 @@ def __init__(
         # the client will set `response.next_request`.
         self.next_request: typing.Optional[Request] = None
 
-        self.extensions = {} if extensions is None else extensions
+        self.extensions: ResponseExtensions = {} if extensions is None else extensions
         self.history = [] if history is None else list(history)
 
         self.is_closed = False