[Request] Provide source distribution

Currently, there is [no source distribution](https://pypi.org/project/django-cryptography/#files) on PyPi. It would be nice to have for [Nix](https://nixos.org) users.

<img width="516" alt="Screen Shot 2022-08-24 at 16 18 18" src="https://user-images.githubusercontent.com/543423/186515583-a4523f96-8b82-493d-8e3b-62507ca4aafe.png">