Adapt to changed function name in Python 3.13

[ellert]

Description

According to https://docs.python.org/3.13/whatsnew/3.13.html:

Add PyThreadState_GetUnchecked() function: similar to PyThreadState_Get(), but don't kill the process with a fatal error if it is NULL. The caller is responsible to check if the result is NULL. Previously, the function was private and known as _PyThreadState_UncheckedGet().

As part of preparing for updating Python to version to 3.13 in Fedora 41 (scheduled for October 2024) rebuilds of existing Python packages are attempted against the current Python 3.13 version in a separate development repository. Currently this is Python 3.13.0a1.

F41 Change Proposal: Python 3.13 (System-Wide)

As a maintainer of packages in Fedora, I got two bugs filed against packages I maintain that failed to build in this rebuiild due to the issue this PR proposes to address:

• HepMC3 fails to build with Python 3.13: error: ‘_PyThreadState_UncheckedGet’ was not declared in this scope; did you mean ‘PyThreadState_GetUnchecked’?
• pythia8 fails to build with Python 3.13: error: ‘_PyThreadState_UncheckedGet’ was not declared in this scope

With the proposed change these packages can be built with the Python 3.13.0a1 from the special development repo.

[rwgk]

Thanks! Could you please add a sentence or two to the PR description: How did you test this (platform(s), CPython git hash)? Maybe if interesting: why/context? — This is mainly so others can gauge: might it work for me, too?

Similarly, do you think we could test here (pybind11 CI) already, too? Or is it still too early? (I'd want to be around the center of the earlier-adopter - to - late adopter bell curve.)

[rwgk]

Forgot to add: Please tag me here after updating the PR description, to be sure I don't miss it.

[ellert]

Forgot to add: Please tag me here after updating the PR description, to be sure I don't miss it.

@rwgk : I have uodated the description as requested.

[rwgk]

Awesome, thanks!