fix: now sanctuary destroys previous secret versions

See #26 (closed)

Fix explanation:

Current implementation - if getattr(version, "destroy_time", None) is None: is False every time and versions destroying simply doesn't work, because attribute destroy_time is always there. Easy to check it with hasattr(version, "destroy_time"). This is always True for every item in secret_versions.versions[1:]. It seems this is a google's lib behaviour.

So, as said, destroy_time attribute is always there, (type: <class 'google.protobuf.timestamp_pb2.Timestamp'>). The only difference is that the object field "seconds" is 0 (int) for non-destroyed versions (because destroy_time.seconds indicates time since version was destroyed).

So that's why the easiest way was to check if it is == 0 and if it is, destroy the version.