Adapt to changed function name in Python 3.13 (#4902)

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().