Improve security in pickle module

Fix a potential denial of service in the :mod:`pickle` module by preventing arbitrary memory allocation when unpickling data from untrusted sources.

Author: serhiy-storchaka

Misc/NEWS.d/next/Library/2024-05-20-12-35-52.gh-issue-115952.J6n_Kf.rst

@@ -1,2 +1,8 @@
-:mod:`pickle` now avoids allocating an arbitrary large amount of memory
-based on small untrusted input when unpickling specially crafted data.
+Fix a potential denial of service in the :mod:`pickle` module.
+When reading a pickled data received from untrusted source, it could cause
+an arbitrary amount of memory to be allocated, even if the code that is
+allowed to execute is restricted by overriding the
+:meth:`~pickle.Unpickler.find_class` method.
+In best case this could lead to a :exc:`MemoryError` or other process crash.
+In worst case it could lead to swapping which would dramatically slow down the
+whole system and make it less responcible.