Calling PyEval_ReleaseLock() was crashing with Python 3.9
because it accessed the NULL pointer set by PyThreadState_Swap().
This happened when calling ViewLayer.update() for example.
While the existing logic could be fixed by swapping the thread-state
back before calling PyEval_ReleaseLock(), this depends on functions
which are tagged to be removed by v4.0.
Replace use of deprecated functions by calling PyEval_SaveThread(),
instead of inlining the logic, using _PyThreadState_UncheckedGet()
to prevent Python aborting.
The call to PyEval_ThreadsInitialized has been removed
as threads are now initialized with Python.
This could be replaced with Py_IsInitialized() however it doesn't look
like this is necessary.
This is compatible with Python 3.7 & 3.9.
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