Fixes #23643 - Show exception chain in debug template for Py3k

https://code.djangoproject.com/ticket/23643#no4
Maybe you can extract the test code from the Py3ExceptionReporterTests and skip importing the "actual test code" if six.PY3 is False?

```python
class Py3ExceptionReporterTests(TestCase):
    rf = RequestFactory()
    @unittest.skipIf(not six.PY3, "Python 3 only test")
    def test_reporting_of_nested_exceptions(self):
        do_actual_testing(self.rf)
```