bpo-1635741: Clean sysdict and builtins of interpreter at exit (GH-21605)

Author: shihai1991

Python/pystate.c

@@ -294,8 +294,6 @@ PyInterpreterState_Clear(PyInterpreterState *interp)
  Py_CLEAR(interp->codec_error_registry);
  Py_CLEAR(interp->modules);
  Py_CLEAR(interp->modules_by_index);
- Py_CLEAR(interp->sysdict);
- Py_CLEAR(interp->builtins);
  Py_CLEAR(interp->builtins_copy);
  Py_CLEAR(interp->importlib);
  Py_CLEAR(interp->import_func);
@@ -308,6 +306,14 @@ PyInterpreterState_Clear(PyInterpreterState *interp)
  if (_PyRuntimeState_GetFinalizing(runtime) == NULL) {
  _PyWarnings_Fini(interp);
  }
+ /* We don't clear sysdict and builtins until the end of this function.
+ Because clearing other attributes can execute arbitrary Python code
+ which requires sysdict and builtins. */
+ PyDict_Clear(interp->sysdict);
+ PyDict_Clear(interp->builtins);
+ Py_CLEAR(interp->sysdict);
+ Py_CLEAR(interp->builtins);
+
  // XXX Once we have one allocator per interpreter (i.e.
  // per-interpreter GC) we must ensure that all of the interpreter's
  // objects have been cleaned up at the point.