Fixed #23923 -- Promoted Django's deprecation warnings to errors in runtests.py

As the :ref:`deprecation policy<internal-release-deprecation-policy>` describes,

the first release of Django that deprecates a feature (``A.B``) should raise a

``RemovedInDjangoXXWarning`` (where XX is the Django version where the feature

will be removed) when the deprecated feature is invoked. Assuming

we have a good test coverage, these warnings will be shown by the test suite

when :ref:`running it <running-unit-tests>` with warnings enabled:

``python -Wall runtests.py``. This is annoying and the output of the test suite

should remain clean. Thus, when adding a ``RemovedInDjangoXXWarning`` you need

to eliminate or silence any warnings generated when running the tests.

will be removed) when the deprecated feature is invoked. Assuming we have good

test coverage, these warnings are converted to errors when :ref:`running the

test suite <running-unit-tests>` with warnings enabled:

``python -Wall runtests.py``. Thus, when adding a ``RemovedInDjangoXXWarning``

you need to eliminate or silence any warnings generated when running the tests.

The first step is to remove any use of the deprecated behavior by Django itself.

Next you can silence warnings in tests that actually test the deprecated

    import warnings

    from django.utils.deprecation import RemovedInDjangoXXWarning

    def test_foo(self):

        with warnings.catch_warnings(record=True) as w:

            warnings.simplefilter("always")

        with warnings.catch_warnings():

            warnings.simplefilter("ignore", category=RemovedInDjangoXXWarning)

            # invoke deprecated behavior

        # go ahead with the rest of the test

    class MyDeprecatedTests(IgnorePendingDeprecationWarningsMixin, unittest.TestCase):

        ...

You can also add a test for the deprecation warning. You'll have to disable the

"warning as error" behavior in your test by doing::

    import warnings

    def test_foo_deprecation_warning(self):

        with warnings.catch_warnings(record=True) as warns:

            warnings.simplefilter('always')  # prevent warnings from appearing as errors

            # invoke deprecated behavior

        self.assertEqual(len(warns), 1)

        msg = str(warns[0].message)

        self.assertEqual(msg, 'Expected deprecation message')

Finally, there are a couple of updates to Django's documentation to make:

#) If the existing feature is documented, mark it deprecated in documentation

        iaf2 = InlineAdminForm(None, None, {}, {}, poll)

        poll_ct = ContentType.objects.get_for_model(Poll)

        with warnings.catch_warnings(record=True) as recorded:

            warnings.filterwarnings('always')

            with self.assertRaises(AttributeError):

                iaf.original_content_type_id

            msg = force_text(recorded.pop().message)

)

# suppress the deprecation warning of memoize

with warnings.catch_warnings(record=True):

with warnings.catch_warnings():

    warnings.filterwarnings('ignore')

    fully_decorated = memoize(fully_decorated, {}, 1)

fully_decorated = full_decorator(fully_decorated)

                pass

        with warnings.catch_warnings(record=True) as recorded:

            warnings.filterwarnings('always')

            suite = unittest.TestLoader().loadTestsFromTestCase(TempTestCase)

            with open(os.devnull, 'w') as devnull:

                unittest.TextTestRunner(stream=devnull, verbosity=2).run(suite)

    def test_get_form_missing_form_class_default_value(self):

        with warnings.catch_warnings(record=True) as w:

            warnings.filterwarnings('always')

            class MissingDefaultValue(FormMixin):

                request = RequestFactory().get('/')

                form_class = forms.Form