Update publish / release / CD workflow

[sgraaf]

Update publish / release / CD workflow such that:

• The package is built and checked on every push
• The package is published to TestPyPI on every push (optional)
• The package is published to PyPI only when a tag is pushed

Some examples that could serve as a good starting point:

• https://github.com/pradyunsg/furo/blob/main/.github/workflows/release.yml
• https://github.com/hynek/svcs/blob/main/.github/workflows/pypi-package.yml
• https://github.com/astral-sh/ruff/blob/main/.github/workflows/release.yaml