Fix extensions type annotation.

httpx/_models.py

@@ -398,7 +398,7 @@ def __init__(
         self.method = method.upper()
         self.url = URL(url) if params is None else URL(url, params=params)
         self.headers = Headers(headers)
-        self.extensions = {} if extensions is None else extensions
+        self.extensions = {} if extensions is None else dict(extensions)
 
         if cookies:
             Cookies(cookies).set_cookie_header(self)
@@ -537,7 +537,7 @@ def __init__(
         # the client will set `response.next_request`.
         self.next_request: Request | None = None
 
-        self.extensions: ResponseExtensions = {} if extensions is None else extensions
+        self.extensions = {} if extensions is None else dict(extensions)
         self.history = [] if history is None else list(history)
 
         self.is_closed = False

httpx/_types.py

@@ -15,7 +15,6 @@
     Iterator,
     List,
     Mapping,
-    MutableMapping,
     Optional,
     Sequence,
     Tuple,
@@ -67,7 +66,7 @@
 
 RequestContent = Union[str, bytes, Iterable[bytes], AsyncIterable[bytes]]
 ResponseContent = Union[str, bytes, Iterable[bytes], AsyncIterable[bytes]]
-ResponseExtensions = MutableMapping[str, Any]
+ResponseExtensions = Mapping[str, Any]
 
 RequestData = Mapping[str, Any]
 
@@ -84,7 +83,7 @@
 ]
 RequestFiles = Union[Mapping[str, FileTypes], Sequence[Tuple[str, FileTypes]]]
 
-RequestExtensions = MutableMapping[str, Any]
+RequestExtensions = Mapping[str, Any]
 
 __all__ = ["AsyncByteStream", "SyncByteStream"]