CI: Add PyPI attestations

Benjamin Bertrandrequested to merge
pypi-attestations into develop

After reading https://snarky.ca/why-pylock-toml-includes-digital-attestations/, I think it's a good idea to add digital attestations to pytango release on PyPI.

I followed https://docs.pypi.org/attestations/producing-attestations/#gitlab-cicd

The generate-pypi-attestations job is creating .publish.attestation files. The original wheels aren't modified.

Removed the old image stage that isn't used anymore.

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