[lldb] Adapt code to Python 3.13 (#70445)

1. Remove usage of PyEval_ThreadsInitialized and PyEval_InitThreads

Both of these functions were removed in Python 3.13 [1] after being
deprecated since Python 3.9.

According to "What's new in Python 3.13" document [1]:

    Since Python 3.7, Py_Initialize() always creates the GIL: calling
    PyEval_InitThreads() did nothing and PyEval_ThreadsInitialized()
    always returned non-zero.

2. Replace _Py_IsFinalizing() with Py_IsFinalizing().

[1] https://docs.python.org/3.13/whatsnew/3.13.html