Debian packages unconditionally byte-compile .py files on installation and
do not silence errors by design. Therefore, we need a way of shipping these
invalid .py files without a .py extension but ensuring that when we
template them, they end up as .py.

We don't special-case .py files so that the all the TemplateCommand
command-line options (eg. extra_files and extensions) still work entirely
as expected and it may even be useful for other formats too.

This makes it possible to run django.setup() in management commands that
don't need a settings module. In addition it simplifies error handling.

Thanks Claude for the review.

Added comments in the three empty models.py files that are still needed.

Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.

* Removed trailing whitespace.
* Added newline to EOF if missing.
* Removed blank lines at EOF.
* Removed some stray tabs.