Add GitHub Actions workflow for automated PyPI publishing

[robcohen]

Summary

This adds a GitHub Actions workflow that automatically publishes to PyPI when you create a release.

Background

I needed ibflex for a project but the PyPI version (0.15, Jan 2021) is missing years of updates. Rather than just complain, I set up CI and published a fork as ibflex2 to unblock myself and others.

This PR

Adds .github/workflows/publish.yml that:

• Triggers on GitHub release creation
• Builds the package using python -m build
• Publishes to PyPI using trusted publishing (no tokens needed)

Setup Required

To use this, you'll need to:

1. Create a GitHub environment called pypi (Settings → Environments → New environment)
2. Configure "trusted publishing" on PyPI:
   • Go to https://pypi.org/manage/project/ibflex/settings/publishing/
   • Add a trusted publisher with:
     • Owner: csingley
     • Repository: ibflex
     • Workflow: publish.yml
     • Environment: pypi
3. Create a release and it will auto-publish

If You'd Prefer Not To

No worries - ibflex2 exists on PyPI for those who need current updates. If you do start publishing regular releases, I'm happy to deprecate the fork.

Closes #73