bpo-43908: Add What's New entry for Py_TPFLAGS_IMMUTABLETYPE flag (GH-25816)

Author: Erlend Egeberg Aasland

Doc/whatsnew/3.10.rst

@@ -1846,6 +1846,10 @@ New Features
   creating type instances.
   (Contributed by Victor Stinner in :issue:`43916`.)
 
+* Add a new :c:data:`Py_TPFLAGS_IMMUTABLETYPE` type flag for creating immutable
+  type objects: type attributes cannot be set nor deleted.
+  (Contributed by Victor Stinner and Erlend E. Aasland in :issue:`43908`.)
+
 Porting to Python 3.10
 ----------------------
 
@@ -1903,6 +1907,12 @@ Porting to Python 3.10
   been included directly, consider including ``Python.h`` instead.
   (Contributed by Nicholas Sim in :issue:`35134`)
 
+* Use the :c:data:`Py_TPFLAGS_IMMUTABLETYPE` type flag to create immutable type
+  objects. Do not rely on :c:data:`Py_TPFLAGS_HEAPTYPE` to decide if a type
+  object is mutable or not; check if :c:data:`Py_TPFLAGS_IMMUTABLETYPE` is set
+  instead.
+  (Contributed by Victor Stinner and Erlend E. Aasland in :issue:`43908`.)
+
 Deprecated
 ----------