Publish policyengine-taxsim to PyPI

[MaxGhenis]

Summary

policyengine-taxsim is not currently on PyPI. Users must install from GitHub, which is cumbersome.

Publishing to PyPI would allow a simple one-liner install. This would also make the R package's Config/reticulate declaration simpler (#722), and make the installation instructions on the landing page cleaner.

Steps

1. Verify pyproject.toml has correct metadata (name, version, description, classifiers)
2. Set up PyPI API token as a GitHub secret
3. Add a GitHub Actions workflow to publish on release/tag
4. Update install instructions in README, landing page, R package, and docs