Draft: ci: Use PyPI Trusted Publishers

• Use PyPI Trusted Publishers to publish from GitLab CI/CD to TestPyPI and PyPI. c.f.:
  • https://blog.pypi.org/posts/2024-04-17-expanding-trusted-publisher-support/
  • https://docs.pypi.org/trusted-publishers/using-a-publisher/#gitlab-cicd

Blocking: Requires https://docs.pypi.org/trusted-publishers/adding-a-publisher/ to happen first.