Also change headers attribute type

Author: intgr

django-stubs/test/client.pyi

@@ -188,7 +188,7 @@ class Client(ClientMixin, _RequestFactory[_MonkeyPatchedWSGIResponse]):
     raise_request_exception: bool
     exc_info: tuple[type[BaseException], BaseException, TracebackType] | None
     extra: dict[str, Any] | None
-    headers: dict[str, Any]
+    headers: Mapping[str, str] | None
     def __init__(
         self,
         enforce_csrf_checks: bool = ...,
@@ -278,7 +278,7 @@ class AsyncClient(ClientMixin, _AsyncRequestFactory[Awaitable[_MonkeyPatchedASGI
     raise_request_exception: bool
     exc_info: Any
     extra: dict[str, Any] | None
-    headers: dict[str, Any]
+    headers: Mapping[str, str] | None
     def __init__(
         self,
         enforce_csrf_checks: bool = ...,