Publish to PyPI

[gotnull]

The tool is already pip-installable via git+https://... but publishing to PyPI would make it a simple:

pip install likelihoodlum
likelihoodlum owner/repo

Tasks:

• Register the likelihoodlum name on PyPI
• Add a GitHub Actions workflow for automated publishing on release
• Add a PyPI badge to the README
• Test install from PyPI in a clean environment