gh-151046: Fix use-after-free in _Unpickler_ReadIntoFromFile

[tonghuaroot]

When the C pickle.Unpickler reads from a file-like object with readinto(),
it wraps an internal, short-lived buffer in a memoryview and passes it to
readinto(). The view was never released after the call, so a readinto()
that retains the view could read or write the buffer after it was freed -- a
C-level use-after-free reachable from pure Python.

This keeps our own reference across the call (using PyObject_CallOneArg
instead of the reference-stealing _Pickle_FastCall) and releases the
memoryview as soon as readinto() returns. A surviving reference now raises
ValueError: operation forbidden on released memoryview object instead of
touching freed memory.

A well-behaved readinto() releases any view it acquired before it returns, so
by then the buffer's export count is already zero and the added release() is a
no-op -- ordinary file objects round-trip unchanged. The release only has an
effect when a readinto() improperly keeps the view alive. The readinto()
fast path dates back to bpo-39681, so the issue is present on all maintained
branches.

Adds a regression test (CUnpicklerTests.test_readinto_does_not_keep_buffer_alive)
that fails on the unpatched build and a Misc/NEWS.d entry.

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• Issue: Use-after-free in _Unpickler_ReadIntoFromFile: temporary memoryview passed to readinto() can outlive its buffer #151046