bpo-29587: Enable implicit exception chaining with gen.throw() (GH-19811)

Before this commit, if an exception was active inside a generator
when calling gen.throw(), then that exception was lost (i.e. there
was no implicit exception chaining).  This commit fixes that.

Lib/test/test_generators.py

@@ -316,6 +316,23 @@ class ExceptionTest(unittest.TestCase):
         self.assertEqual(cm.exception.value.value, 2)
 
 
+class GeneratorThrowTest(unittest.TestCase):
+
+    def test_exception_context_set(self):
+        def f():
+            try:
+                raise KeyError('a')
+            except Exception:
+                yield
+
+        gen = f()
+        gen.send(None)
+        with self.assertRaises(ValueError) as cm:
+            gen.throw(ValueError)
+        context = cm.exception.__context__
+        self.assertEqual((type(context), context.args), (KeyError, ('a',)))
+
+
 class YieldFromTests(unittest.TestCase):
     def test_generator_gi_yieldfrom(self):
         def a():