Fix a potential memory 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.
This could have led to symptoms including a :exc:`MemoryError`, swapping, out
of memory (OOM) killed processes or containers, or even system crashes.
