bpo-43908: Add Py_TPFLAGS_IMMUTABLETYPE flag (GH-25520)

Introduce Py_TPFLAGS_IMMUTABLETYPE flag for immutable type objects, and
modify PyType_Ready() to set it for static types.

Co-authored-by: Victor Stinner <vstinner@python.org>
diff --git a/Include/object.h b/Include/object.h
index 695f015..d8476f9 100644
--- a/Include/object.h
+++ b/Include/object.h
@@ -320,6 +320,9 @@ Code can use PyType_HasFeature(type_ob, flag_value) to test whether the
 given type object has a specified feature.
 */
 
+/* Set if the type object is immutable: type attributes cannot be set nor deleted */
+#define Py_TPFLAGS_IMMUTABLETYPE (1UL << 8)
+
 /* Set if the type object is dynamically allocated */
 #define Py_TPFLAGS_HEAPTYPE (1UL << 9)