Restore _PyLong_NumBits on Python 3.13 and later

[exg]

See python/cpython@cc663b7

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[davidhewitt]

Xref #4379

While the general trend is to take _Py symbols out of the public API, in this case the CPython devs seem to have agreed to keep this around for now (and I was planning to re-expose in #4379 too).

Thanks 👍

[exg]

@davidhewitt I have been waiting for a new pyo3 release with this change in order to make a new ormsgpack release with python 3.13 support. Alas, pyo3 0.22.3 does not include it. Would it by chance be possible to backport the change to the release-0.22 branch and make a new release?

[davidhewitt]

Thanks for the ping. I think I will need to put out a 0.22.4 patch with #4563 so I will include this at the same time.