Migrate packaging configuration to pyproject.toml

[fcooper8472]

I suggest a full migration from the legacy setup.py packaging configuration to a modern pyproject.toml-based setup.

Planned changes

• Move package metadata from setup.py into pyproject.toml
• Add a proper [build-system] section
• Declare dependencies in pyproject.toml
• Add explicit Python version support via requires-python
• Add classifiers (so supported versions appear on PyPI)

One benefit would be seeing at a glance on PyPI which Python versions are currently supported. @kwabenantim I can do this work on a fork and open a PR, but please let me know if there's a reason not to make this migration.

[kwabenantim]

This sounds good to me @fcooper8472.

[kwabenantim]

@fcooper8472 I can have a go at this now if that's okay! I won't start a PR if you've got work in progress already.