Add GitHub Actions workflow for automated PyPI publishing

This adds a workflow that automatically publishes to PyPI when a release is created.

Setup required:
1. Create a GitHub environment called "pypi"
2. Configure trusted publishing on PyPI at:
   https://pypi.org/manage/project/ibflex/settings/publishing/
   - Owner: csingley
   - Repository: ibflex
   - Workflow: publish.yml
   - Environment: pypi

Closes #73