Upgrade packaging to use Flit.

[carltongibson]

Message from PyPI uploading 1.1.0:

In the future, PyPI will require all newly uploaded source distribution filenames to comply with PEP 625. Any source distributions already uploaded will remain in place as-is and do not need to be updated.

Specifically, your recent upload of 'django-appconf-1.1.0.tar.gz' is incompatible with PEP 625 because it does not contain the normalized project name 'django_appconf'.

In most cases, this can be resolved by upgrading the version of your build tooling to a later version that supports PEP 625 and produces compliant filenames. You do not need to remove the file.

Time to move from setup.py anyway I guess. Let's use Flit