Issue 22906: Add test file.

Lib/test/test_pep479.py

+from __future__ import generator_stop
+
+import unittest
+
+
+class TestPEP479(unittest.TestCase):
+    def test_stopiteration_wrapping(self):
+        def f():
+            raise StopIteration
+        def g():
+            yield f()
+        with self.assertRaisesRegex(RuntimeError,
+                                    "generator raised StopIteration"):
+            next(g())
+
+    def test_stopiteration_wrapping_context(self):
+        def f():
+            raise StopIteration
+        def g():
+            yield f()
+
+        try:
+            next(g())
+        except RuntimeError as exc:
+            self.assertIs(type(exc.__cause__), StopIteration)
+            self.assertIs(type(exc.__context__), StopIteration)
+            self.assertTrue(exc.__suppress_context__)
+        else:
+            self.fail('__cause__, __context__, or __suppress_context__ '
+                      'were not properly set')
+
+
+if __name__ == '__main__':
+    unittest.main()