Add a "What's New" entry.

Author: ericsnowcurrently

Doc/whatsnew/3.12.rst

@@ -2236,6 +2236,15 @@ Porting to Python 3.12
   guidance then please create a new GitHub issue
   and CC ``@ericsnowcurrently``.
 
+  * The :mod:`threading` module now expects the :mod:`!_thread` module to have
+    a ``_is_main_interpreter`` attribute.  Is is a function with no
+    arguments that returns ``True`` if the current interpreter is the
+    main interpreter.
+
+    Any library or application that provides a custom ``_thread`` module
+    must make sure it provides ``_is_main_interpreter()``.
+    (See :gh:`112826`.)
+
 Deprecated
 ----------
 

Doc/whatsnew/3.13.rst

@@ -1046,6 +1046,15 @@ Changes in the Python API
   retrieve a username, instead of :exc:`ImportError` on non-Unix platforms or
   :exc:`KeyError` on Unix platforms where the password database is empty.
 
+  * The :mod:`threading` module now expects the :mod:`!_thread` module to have
+    a ``_is_main_interpreter`` attribute.  Is is a function with no
+    arguments that returns ``True`` if the current interpreter is the
+    main interpreter.
+
+    Any library or application that provides a custom ``_thread`` module
+    must make sure it provides ``_is_main_interpreter()``.
+    (See :gh:`112826`.)
+
 
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