Switch to pyproject.toml over setup.py

[sethmlarson]

• Switches to using Flit instead of setuptools to build the packages due to being pure Python
• Switches to pyproject.toml for static packaging metadata
• Adds the Python 3.11 classifier

[jribbens]

Why is this a good thing?

[kjd]

I'll admit to not being up-to-date enough on the state of the art of packaging to know if this desirable at this point. I think this project values backward compatibility so my initial concern is any unintended side-effects of making this change.

I am very happy to be educated otherwise I will do some reading!

[sethmlarson]

@kjd @jribbens Great questions!

Why pyproject.toml?

This file describes the build system in use, so regardless of whether we accept Flit or moving packaging metadata to a static file (setup.py -> pyproject.toml) we should probably have this file to conform with current standards and get build isolation. Here's a longer explanation about pyproject.toml.

Why Flit?

Flit is a PEP 517 compliant builder that is optimized for pure Python packages that have <module>.__version__ which fits the bill pretty well for idna. There shouldn't be any breakage or changes to installers, only builders (us and OS re-packagers downstream) will need to do anything. If it's decided we want to stick with setuptools that would be fine too.