gh-112826: Add a "What's New" Entry About _thread._is_main_interpreter (gh-112853)

As of gh-112661, the threading module expects the _thread module to have a _is_main_interpreter(), which is used in the internal threading._shutdown().  This change causes a problem for anyone that replaces the _thread module with a custom one (only if they don't provide _is_main_interpreter()).  They need to be sure to add it for 3.13+, thus this PR is adding a note in "What's New".

This also forward-ports the "What's New" entry from 3.12 (gh-112850).  Note that we do not also forward-port the fix in that PR.  The fix is there only due to a regression from 3.12.0. There is no regression in 3.13+.

Doc/whatsnew/3.12.rst

@@ -1895,6 +1895,15 @@ 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

@@ -1082,6 +1082,16 @@ 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`.)
+
 
 Build Changes
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