bpo-40696: Fix a hang that can arise after gen.throw() (GH-20287)

This updates _PyErr_ChainStackItem() to use _PyErr_SetObject()
instead of _PyErr_ChainExceptions(). This prevents a hang in
certain circumstances because _PyErr_SetObject() performs checks
to prevent cycles in the exception context chain while
_PyErr_ChainExceptions() doesn't.

Include/internal/pycore_pyerrors.h

@@ -51,7 +51,7 @@ PyAPI_FUNC(void) _PyErr_SetObject(
     PyObject *value);
 
 PyAPI_FUNC(void) _PyErr_ChainStackItem(
-    _PyErr_StackItem *exc_state);
+    _PyErr_StackItem *exc_info);
 
 PyAPI_FUNC(void) _PyErr_Clear(PyThreadState *tstate);