fix: include django-extensions as required installations

[simkimsia]

Because autoslugfield requires django-extensions as indicated here https://github.com/bennylope/django-organizations/blob/2dd044956cf03dad9c8c4954ba325d2769fe3643/docs/getting_started.rst#auto-slug-field

and the minimum version is taken from

django-organizations/requirements-test.txt

Line 10 in 2dd0449

django-extensions>=2.0.8

[bennylope]

I'm not overly fond of including this by default, since you can use the package as a "plain" library without needing to install the Django app (thus not needing this dependency).

However the alternatives are (1) status quo or (2) declare named optional dependencies in the setup files... this is probably the simplest way forward. Thank you!