This would break upcoming changes and AdminSite assumptions about
having an app_config for each application that has registered
models.

Refs f8e8853b

Reflecting the unfortunate reality.
Still plan to be involved, but feel I need to recognize the degree to
which I'm only on the periphery these days.

Instead of setting ``warning_stream`` in the docutils config overrides
to ``False`` I opted for filtering the stderr in the tests to keep the
error output showing up in server logs.

Thanks Tim Graham for the report and review

Changed SessionBase.pop's second argument to explicitly be default=None
rather than *args since _session is always a dict. Thanks gabor for the
report and Tim Graham for the review.

Thanks Tim Graham for the review.

Thanks Scott Sanders for the report, and Markus Holtermann and
Tim Graham for the reviews. Refs #24381.

Thanks Anssi Kääriäinen for review.

Use mock.patch in migrations tests

Currently some of the migrations tests rely on the fact 'input' is aliased
because of six, instead of using mock.patch.  Replace this code with proper
use of mock.patch.

Also, replace one case of excessively specific python version check with
testing six.PY3

This reverts commit 72f769f4.

There are several test failures that need to be fixed.