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.
(cherry picked from commit 7c30d12bd5)

Co-authored-by: Chris Jerdonek <chris.jerdonek@gmail.com>

Lib/test/test_generators.py

@@ -371,6 +371,32 @@ class GeneratorThrowTest(unittest.TestCase):
         context = cm.exception.__context__
         self.assertEqual((type(context), context.args), (KeyError, ('a',)))
 
+    def test_exception_context_with_yield_from_with_context_cycle(self):
+        # Check trying to create an exception context cycle:
+        # https://bugs.python.org/issue40696
+        has_cycle = None
+
+        def f():
+            yield
+
+        def g(exc):
+            nonlocal has_cycle
+            try:
+                raise exc
+            except Exception:
+                try:
+                    yield from f()
+                except Exception as exc:
+                    has_cycle = (exc is exc.__context__)
+            yield
+
+        exc = KeyError('a')
+        gen = g(exc)
+        gen.send(None)
+        gen.throw(exc)
+        # This also distinguishes from the initial has_cycle=None.
+        self.assertEqual(has_cycle, False)
+
     def test_throw_after_none_exc_type(self):
         def g():
             try: