Added CD: .github/workflows/deploy.yml

[Nusnus]

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[Nusnus]

Woohoo 🥳
We have full CI/CD in pytest-celery.
A release to GitHub is all that is required (even less - just a version tag).

@auvipy I'll explain it all on our zoom meeting, but I am just so happy it worked on the first attempt!
TL;DR for releasing a version:

1. Run poetry version 1.2.3 locally -> commit with 'Bump version: 1.2.3 → 4.5.6'" -> Open PR to main.
2. Once CI is fully passing on the PR -> Merge.
3. Release on github from main with new version tag -> automatically publishes to PyPI.

This gives a double check on the CI in the bump version PR making sure everything is alright before release, also allows adding changelog to this PR and having a single PR for all of the release work. Then, an automatically generated release with a new tag takes care of the CD, and no funny games - it just works :)