docs: def create(cls, object_version, key, value):
          """Create a new tag for a given object version."""
          assert len(key) < 256
          assert len(value) < 256
          with db.session.begin_nested():
              obj = cls(version_id=as_object_version_id(object_version),
                        key=key,
                        value=value)
              db.session.add(obj)
          return obj