gh-112182: Replace StopIteration with RuntimeError for future (#113220)

When an `StopIteration` raises into `asyncio.Future`, this will cause
a thread to hang. This commit address this by not raising an exception
and silently transforming the `StopIteration` with a `RuntimeError`,
which the caller can reconstruct from `fut.exception().__cause__`

Lib/test/test_asyncio/test_futures.py

@@ -270,10 +270,6 @@ class BaseFutureTests:
         f = self._new_future(loop=self.loop)
         self.assertRaises(asyncio.InvalidStateError, f.exception)
 
-        # StopIteration cannot be raised into a Future - CPython issue26221
-        self.assertRaisesRegex(TypeError, "StopIteration .* cannot be raised",
-                               f.set_exception, StopIteration)
-
         f.set_exception(exc)
         self.assertFalse(f.cancelled())
         self.assertTrue(f.done())
@@ -283,6 +279,25 @@ class BaseFutureTests:
         self.assertRaises(asyncio.InvalidStateError, f.set_exception, None)
         self.assertFalse(f.cancel())
 
+    def test_stop_iteration_exception(self, stop_iteration_class=StopIteration):
+        exc = stop_iteration_class()
+        f = self._new_future(loop=self.loop)
+        f.set_exception(exc)
+        self.assertFalse(f.cancelled())
+        self.assertTrue(f.done())
+        self.assertRaises(RuntimeError, f.result)
+        exc = f.exception()
+        cause = exc.__cause__
+        self.assertIsInstance(exc, RuntimeError)
+        self.assertRegex(str(exc), 'StopIteration .* cannot be raised')
+        self.assertIsInstance(cause, stop_iteration_class)
+
+    def test_stop_iteration_subclass_exception(self):
+        class MyStopIteration(StopIteration):
+            pass
+
+        self.test_stop_iteration_exception(MyStopIteration)
+
     def test_exception_class(self):
         f = self._new_future(loop=self.loop)
         f.set_exception(RuntimeError)