converters: Preserve GPG signatures on releases

Since version 0.19.10 (more specifically, this commit:
https://github.com/dulwich/dulwich/commit/72aec0c79fb395689e40f9228df93e2a39cf8fb0),
Dulwich strips GPG signatures from the 'message' attribute of Tag objects,
and stores it in a new attribute, 'signature'.

This means we were silently dropping all signatures from releases.