gh-92810: Add What's New entry

Author: dolfinus

Doc/whatsnew/3.15.rst

@@ -375,10 +375,18 @@ zlib
 Optimizations
 =============
 
-module_name
------------
+abc
+---
+
+* Reduce memory usage of `issubclass` checks for classes inheriting abstract classes.
+
+  :class:`abc.ABCMeta` hook ``__subclasscheck__`` now includes
+  a guard which is triggered then the hook is called from a parent class
+  (``issubclass(cls, RootClass)`` -> ``issubclass(cls, NestedClass)`` -> ...).
+  This guard prevents adding ``cls`` to ``NestedClass`` positive and negative caches,
+  preventing memory bloat in some cases (thousands of classes inherited from ABC).
 
-* TODO
+  (Contributed by Maxim Martynov in :gh:`92810`.)