[1.9.x] Fixed #25968 -- Changed project/app templates to use a "py-tpl" suffix.

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.

Backport of abc0777b from master