[3.14] gh-156995: Fix _PyBytes_Resize() in Free Threading (#156996)

[vstinner]

On Free Threading, _PyBytes_Resize(&obj, 1) no longer returns a single byte singleton is the current thread is different than the thread which created the object.

(cherry picked from commit fd569ea)

• Issue: bytearray.take_bytes() can corrupt shared single-byte bytes objects #156995

[vstinner]

I failed to find a function in Python 3.14 stdlib which still mutates the bytes object after calling _PyBytes_Resize(). The common pattern is more like the following code which is not affected:

bytes = PyBytes_FromStringAndSize(NULL, size);
... write into bytes ...
_PyBytes_Resize(&bytes, n);
return bytes;  // bytes no longer mutated from this point

There is _io.BytesIO.truncate() which may be affected by the issue, but its resize function allocates "size + 1" and so is not affected at the end.

[vstinner]

This change should be backported to Python 3.13, but the fix there is different because _PyBytes_Resize() doesn't test _PyObject_IsUniquelyReferenced() but tests Py_REFCNT(v) == 1 instead.

[bedevere-app]

GH-157436 is a backport of this pull request to the 3.14 branch.