Update package specification and build system

[DavidStirling]

Key changes:

• Migrate to pyproject.toml project specification format
• Revise GH actions workflow using the newer template
• Replace version.py with setuptools-scm
• Drop support for EOL Python versions (3.6-3.8)

[sbesson]

The migration of the package to use pyproject.toml is consistent with the steps taken in other projects.

At the CI level, only consideration is whether the reduction of the test matrix to a single Python version on Linux can reduce coverage. I'll leave @emilroz to comment on the latter as he originally introduced the workflow YAML file.

[DavidStirling]

Thanks Seb. I opted to drop the matrix primarily because we don't actually have a test suite and our wheels are cross-platform. In effect all we're really doing with the current system is running flake8 on several different Python versions.

[emilroz]

At the CI level, only consideration is whether the reduction of the test matrix to a single Python version on Linux can reduce coverage. I'll leave @emilroz to comment on the latter as he originally introduced the workflow YAML file.

Would it make sense to at least cover 3.10 & 3.12?