chore: Adopt pyproject.toml

[effigies]

This is a straight translation of setup.py to pyproject.toml using:

uvx setuptools-py2cfg >> setup.cfg
uvx 'ini2toml[full]' -o pyproject.toml setup.cfg

Two changes needed:

1. Add content-type to project.readme table.
2. Remove Download URL, since interpolating the version string isn't feasible. I added a link to the GitHub as an alternative.

The main reason I did this was because the presence of requires-python in pyproject.toml is used by several tools to determine what features can be assumed when linting a project.