Consider moving FlatPageSitemap to contrib.flatpages

[Keryn Knight]

The FlatPageSitemap currently exists within the 'core' django.contrib.sitemaps namespace, though it's specific to django.contrib.flatpages. Given a number of modules and moves have happened recently, I'm floating the idea of:

• Moving FlatPageSitemap to django.contrib.flatpages.sitemaps, which would better reflect it's dependencies (though it still has one to django.contrib.sites), and is more inline with how sitemaps would be packaged in third-party apps (and would allow spinning out flatpages to an external app with less work, if ever desired)
• Raising a deprecation warning for a release, if importing from the old namespace (like the ContentTypes move and form.util moves)

The documentation currently says The sitemap framework provides a couple convenience classes for common cases, and I'd argue that flatpages isn't all *that* common, these days, though it may have been in the past (before richer packages either built on it or supplanted it, because the ecosystem was smaller)

[Berker Peksag]

​https://github.com/django/django/pull/3710

[Tim Graham]

Test failure needs fixing.

[Berker Peksag]

I've fixed the test failures: ​http://djangoci.com/job/django-pull-requests/2003/

[Tim Graham <timograham@…>]

In 9aeb917dad2c3d4c2830f38d3bfc63699e0ba837:

Fixed #23884 -- Moved FlatPageSitemap into django.contrib.flatpages.

[Tim Graham]

Please look at silencing the deprecation warning when running the tests in reverse: ​http://djangoci.com/job/django-master-trusty-reverse/

[Tim Graham <timograham@…>]

In 4efe1b79e9462318ff8b2c76a09c5925ad0e0f7c:

Refs #23884 -- Silenced a deprecation warning in sitemaps tests.

[Tim Graham <timograham@…>]

In c2d5f2903cfaef4f8b17d86f4db706b61073a471:

Removed contrib.flatpages.FlatPageSitemap per deprecation timeline; refs #23884.