event: Don't use _PyGen_FetchStopIterationValue in Python 3.13+

[darktohka]

Issue description

Python 3.13 has unpublished the _PyGen_FetchStopIterationValue private C API call. It is now part of the internal API: python/cpython#106320

Solution description

This pull request replaces the private _PyGen_FetchStopIterationValue API call with the new PyErr_GetRaisedException API call, added in Python 3.12.

Because the versioning is so complicated (PyErr_GetRaisedException being added in Python 3.12, _PyGen_FetchStopIterationValue not working in Python 3.13, older versions need to be supported still) I have decided to do the following:

• Keep the old behaviour on versions older than Python 3.13, as it works perfectly fine
• Use PyErr_GetRaisedException on Python 3.13+

Right now this code is not covered by the test suite, given that Python 3.13 is still in alpha.

Checklist

I have done my best to ensure that…

• …I have familiarized myself with the CONTRIBUTING.md file
• …this change follows the coding style and design patterns of the codebase
• …I own the intellectual property rights to this code
• …the intent of this change is clearly explained
• …existing uses of the Panda3D API are not broken
• …the changed code is adequately covered by the test suite, where possible.

[rdb]

Thanks.

Should we check for whether the stored value may be nullptr? Right now that would result in a crash.

I agree with only enabling this in Python 3.13. Have you run the test suite with the new code enabled on an older version just to make sure it works, though?

[rdb]

Looking at this in more detail, I don't think this fix is right. result also needs to be populated if _function != nullptr, used further down in the function, otherwise there won't be a valid DoneStatus extracted for coroutine tasks:

from direct.directbase import DirectStart
from direct.task.Task import Task
import sys

cont = Task.cont

async def thingy(task):
    print(sys.getrefcount(cont))
    return cont

taskMgr.add(thingy)

base.run()

I'll check something in for 1.10.

[darktohka]

Thank you! I'll let you take things from here then.