fix mypy finding

Author: davidism

src/flask/app.py

@@ -1852,7 +1852,7 @@ def make_response(self, rv: ft.ResponseReturnValue) -> Response:
 
         # extend existing headers with provided headers
         if headers:
-            rv.headers.update(headers)
+            rv.headers.update(headers)  # type: ignore[arg-type]
 
         return rv