Add tox for easy testing of many Python versions

[hugovk]

The tox tool makes it easy to test by taking care of the virtualenv for you and installing dependencies (defined by a tests "extra"). It also makes it easy to test multiple Python versions.

For example:

# Test Python 3.12
tox -e py312

# Test Python 3.12 and pass an argument to pytest
tox -e py312 -- --stepwise

# Test all available Python versions in serial
tox

# Test all available Python versions in parallel
tox -p auto

This PR also:

• Adds support for Python 3.13
• Updates pyproject.toml to use standard metadata instead of tool-specific (flit), so we can add a tests extra to [project.optional-dependencies]
• Adds simple pre-commit config for linting, and to the CI