workflows/release: bump gh-action-sigstore-python, update slug

[woodruffw]

Signed-off-by: William Woodruff william@trailofbits.com

[di]

@woodruffw Not sure why but I can't bring this branch up-to-date, could you do that so we can merge it?

[woodruffw]

Yep, I can do so in a bit.