Add PyUnstable_Exc_PrepReraiseStar

[iritkatriel]

As requested here, it would help cython to have the _PyExc_PrepReraiseStar function exposed in the C API. Since this is an implementation detail (of the except* construct), it can only be exposed in the unstable API.

As per the dev guide, we can still do this in 3.12.

Linked PRs

• gh-105071: add PyUnstable_Exc_PrepReraiseStar to expose except* implementation in the unstable API #105072
• [3.12] gh-105071: add PyUnstable_Exc_PrepReraiseStar to expose except* implementation in the unstable API (GH-105072) #105095
• gh-105071: add missing versionadded directive #105097
• [3.12] gh-105071: add missing versionadded directive (GH-105097) #105105