Align with Django 3.1.6 behaviour for own management wrappers

> commit ec6d2531c59466924b645f314ac33f54470d7ac3
> Author: William Schwartz <wkschwartz@gmail.com>
> Date:   Mon Jan 4 08:50:49 2021 -0600
>
>     Fixed #32314 -- Fixed detection when started non-django modules with "python -m" in autoreloader.
>
>     django.utils.autoreload.get_child_arguments() detected when Python was
>     started with the `-m` option only for `django` module. This commit
>     changes the logic to check __spec__, see
>     https://docs.python.org/3/reference/import.html#main-spec
>
>     Now packages can implement their own __main__ with the runserver
>     command.

Thanks to William for breaking the API in a a.b.c.d.e patch-level
release !