Migrate to trusted publishing

[konstin]

I noticed that I still have a PyPI API token for setuptools-rust. For security hardening, I'd like to remove the token, and instead use trusted publishing exclusively. I can add the trusted publisher if required.

[davidhewitt]

Sucessfully ran a trusted publish in https://github.com/PyO3/setuptools-rust/actions/runs/23583707621/job/68671798019

I have deleted the PYPI_USERNAME and PYPI_PASSWORD secrets from the repo configuration.

[konstin]

Thank you!