This patch moves said check out of the signing loop and performs the
check early in the process. It is then able to report multiple
problems in a single error.
PR #236
This patch adds PEP 740 attestation generation to the workflow: when the Trusted Publishing flow is used, this will generate a publish attestation for each distribution being uploaded. These generated attestations are then fed into `twine`, which newly supports them via `--attestations`.
Ref: https://github.com/pypi/warehouse/issues/15871