`5.3.1` now treats malformed signatures as an error in `ParseUnverified`, which does not align with the documented behavior

5.3.1 now treats malformed signatures as an error here in ParseUnverified which differs from the documented behavior of that method which before ignored signature parsing issues, matching the documented behavior of not validating the signature. This was pushed as a patch to 5.3.1 but feels more like a breaking change that should be published separately, as it changes client expectations for how signatures are treated.