Switch to trusted publishing

[EpicWink]

Trusted publishing (with attestations) means I can know for certain that what I download from PyPI is the same artefact which was generated in GitHub CI, meaning that what I see in GitHub is the same as what is installed - handy for auditing (rather than having to manually review all of the installed files on each release).

See the Python packaging documentation, the PyPI documentation, and the official pypi-publish GitHub action documentation on trusted publishing - you'll need to configure an environment in PyPI and GitHub. You will be able to remove the PYPI_TOKEN project secret.

Because this project already uses the publish GitHub action, there is minimal code change