Fix thread init for Python 3.9+

[cphurley82]

Summary

• Guard the legacy PyEval_ThreadsInitialized() call behind a PY_VERSION_HEX < 0x03090000 check.
• Call PyEval_InitThreads() unconditionally on Python ≥3.9, where PyEval_ThreadsInitialized() was removed, keeping the GIL initialisation behaviour intact.
• Preserve Python ≤3.8 behaviour (only initialise if the threads API reports “not ready”) so older interpreters see no change.

Motivation

Python 3.9 removed PyEval_ThreadsInitialized(), causing PySysC to fail at import/build time on modern interpreters with:
AttributeError: module 'cpython' has no attribute 'PyEval_ThreadsInitialized'

The new guard allows PySysC to run on Python 3.9+ without regressing older Python releases.

[markfoodyburton]

I notice this is marked as a draft, please make sure the commit, when it's ready is signed off by you, but it looks a good patch, thanks.