gitlab-ci: Publish to pypi.org on tag

.gitlab-ci.yml:
Add a publish stage, that uses `poetry publish` on a secure runner to
push a release to pypi.org upon tagging. For this we make use of an
access token configured for the project via the environment variable
`POETRY_PYPI_TOKEN_PYPI`.
3 jobs for issues/4 in 53 seconds (queued for 6 seconds)