Thanks Tom Christie for the report and review.

This is preparation for landing the template-based widget rendering
patch and goes a long way to making these tests more useful for future
development. The old doctest heritage is strong here.

Was added in 44164c5c but the
transaction.commit_unless_managed() line was removed in
ba5138b1; thus the test does nothing
but check that clear() runs.

Using select_related() after .values() or .values_list() is not possible
because .values() or .values_list() already got the values they need.

Thanks to Keryn Knight, Curtis Maloney and Tim Graham for their reviews.

Refs #24704.

With this change, it's expected to survive anything except errors
that make it impossible to import the settings. It's too complex
to fallback to a sensible behavior with a broken settings module.

Harcoding things about runserver in ManagementUtility.execute is
atrocious but it's the only way out of the chicken'n'egg problem:
the current implementation of the autoreloader primarily watches
imported Python modules -- and then a few other things that were
bolted on top of this design -- but we want it to kick in even if
the project contains import-time errors and django.setup() fails.

At some point we should throw away this code and replace it by an
off-the-shelf autoreloader that watches the working directory and
re-runs `django-admin runserver` whenever something changes.

This also fixes a test failure on Python 2 when Django is installed in a
non-ASCII path. This problem cannot happen on Python 3.

* Added helpers to test uncached and cached access.
* Fixed test_project_root_locale: it duplicated test_locale_paths_setting.
* Rewrote test_only_new_files: test more cases.

* When some old files contain errors, the second call to
  gen_filenames() should return them.
* When some new files contain errors, the first call to
  gen_filenames(only_new=True) should return them.

Changed the way makemessages invokes xgettext from one call per
translatable file to one call per locale directory (using --files-from).
This allows to avoid https://savannah.gnu.org/bugs/index.php?35027 and,
as a positive side effect, speeds up localization build.

Previously, messages of ERROR level or higher were printed to
the console.

This speeds up import of 'django.core.validators' which can save a
few hundred milliseconds when importing the module for the first
time. It can be a significant speedup to the django-admin command.

Fixed #25299 -- Fixed crash with ModelAdmin.list_display value that clashes with a model reverse accessor.

Fixed a regression introduced in e1427cc6 when running tests in reverse order.

Introduced an AbstractBaseSession model and hooks providing the option
of overriding the model class used by the session store and the session
store class used by the model.

Moved the lookup in Field.swappable_setting to Apps, and added
an lru_cache to cache the results.

Refs #24743

Thanks Marten Kenbeek for the initial work on the patch. Thanks Aymeric
Augustin and Tim Graham for the review.

Thanks Sayid Munawar and Tim Graham for the report, investigation and
review.