Make a signer release on pypi

[jku]

The release workflow tries to upload to PyPI but PyPI does not accept packages with git dependencies.
so before a PyPI release we need:

• A sigstore release (this is likely very close, there's a release candidate already)
• Securesystemslib update to use the new sigstore API: update SigstoreSigner to sigstore 2.0 secure-systems-lab/securesystemslib#610
• A securesystemslib release

[jku]

Planning to do this release within a week. Blockers:

• test the actions refactor better (hopefully with both GCP and azure online signing)
• Update dependencies (securesystemslib, sigstore) #89

[jku]

This has been fixed a while ago already