Fix T81688: BPY_thread_save crashes with Python 3.9

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.

source/blender/python/generic/bpy_threads.c

@@ -29,14 +29,11 @@
 /* analogue of PyEval_SaveThread() */
 BPy_ThreadStatePtr BPY_thread_save(void)
 {
-  PyThreadState *tstate = PyThreadState_Swap(NULL);
-  /* note: tstate can be NULL when quitting Blender */
-
-  if (tstate && PyEval_ThreadsInitialized()) {
-    PyEval_ReleaseLock();
+  /* The thread-state can be NULL when quitting Blender. */
+  if (_PyThreadState_UncheckedGet()) {
+    return (BPy_ThreadStatePtr)PyEval_SaveThread();
   }
-
-  return (BPy_ThreadStatePtr)tstate;
+  return NULL;
 }
 
 /* analogue of PyEval_RestoreThread() */