The deprecated PyEval_ReleaseLock() triggers a segfault with Python 3.9.0.
See issue at https://developer.blender.org/T81688 and fix at
https://developer.blender.org/rB5edba9b42f684bf8b99894bb6988e7f46180e12c

--- a/source/blender/python/generic/bpy_threads.c	2020-10-13 15:29:16.708347045 -0400
+++ b/source/blender/python/generic/bpy_threads.c	2020-10-13 15:29:43.123345331 -0400
@@ -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() */