gh-112826: Add a "What's New" Entry About _thread._is_main_interpreter

Doc/whatsnew/3.12.rst

@@ -1892,6 +1892,15 @@ Changes in the Python API
   * Mixing tabs and spaces as indentation in the same file is not supported anymore and will
     raise a :exc:`TabError`.
 
+* The :mod:`threading` module now expects the :mod:`!_thread` module to have
+  an ``_is_main_interpreter`` attribute.  It 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
+  should provide ``_is_main_interpreter()``.
+  (See :gh:`112826`.)
+
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Doc/whatsnew/3.13.rst

@@ -1046,6 +1046,16 @@ 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
+  an ``_is_main_interpreter`` attribute.  It 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 provide ``_is_main_interpreter()``, just like the module's
+  other "private" attributes.
+  (See :gh:`112826`.)
+
 
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 =============