[3.10] bpo-43908: Add What's New entry for Py_TPFLAGS_IMMUTABLETYPE flag (GH-25816) (GH-26115) (cherry picked from commit a09fc9c63f1b5980c62ff2712f67500bacb92b04) Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@innova.no> Automerge-Triggered-By: GH:gvanrossum