Add automated PyPI publish workflow

[hownowstephen]

Summary

• Adds .github/workflows/publish.yml — triggered on GitHub releases, builds wheel + sdist, publishes to PyPI via trusted publishing (OIDC)
• Installing the package messes up my projects that have a tests module #90 is already resolved — the packaging modernization in Modernize Python CI and packaging #116 (include = ["customerio*"] + MANIFEST.in prune tests) already excludes tests/ from both wheel and sdist. Verified by inspecting build artifacts.
• publish a wheel #112 root cause: no publish automation existed. Publishing was manual and only uploaded sdist.

Setup required after merge

1. Create a pypi environment in GitHub repo settings
2. Configure trusted publishing on PyPI for customerio/customerio-python → publish.yml → publish job

Closes #112

Test plan

• python -m build produces both .whl and .tar.gz
• Wheel contains only customerio/ package (no tests/)
• Sdist contains only customerio/ package (no tests/)
• Workflow YAML is valid (uses standard pypa/gh-action-pypi-publish@release/v1)

---

Note

Medium Risk
Automates artifact publishing to PyPI on GitHub Release events, so misconfiguration (e.g., trusted publishing/OIDC or Python version availability) could block releases or publish unintended builds.

Overview
Adds a new GitHub Actions workflow (.github/workflows/publish.yml) that triggers on release.published, builds the package (python -m build for wheel + sdist), and publishes to PyPI using pypa/gh-action-pypi-publish with OIDC (id-token: write) in the pypi environment.

^{Reviewed by Cursor Bugbot for commit da689fd. Bugbot is set up for automated code reviews on this repo. Configure here.}