Add Python interpreter option to django_manage.

Enable running `manage.py` with a Python interpreter other than the environment default `python`.

I would prefer to simply run `manage.py` rather than `python manage.py` as `cmd`.  However, *way* back in the day it looks like `manage.py` [was not marked executable and did not have a shebang](https://github.com/django/django/commit/2b50cb8c120d5f4add098fea8ea6fe49ed67dacf) so an option like this will always work.

```sh
----------------------------------------------------------------------
Ran 162 tests in 5.035s

OK
```