gh-139871: Add _PyByteArray_empty_string to the stable ABI

[vstinner]

• Issue: Add .take_bytes([n]) a zero-copy path from bytearray to bytes #139871

[vstinner]

#140128 tried to remove _PyByteArray_empty_string by mistake which would break the stable ABI. Let's put it in the stable ABI to detect if it's removed by mistake.

[encukou]

It's not part of the stable ABI; it's certainly not in the Windows stable ABI DLL.
Let's not add it.

[vstinner]

Oh, it's declared in Include/cpython/bytearrayobject.h, not in Include/bytearrayobject.h, you're right.