Description of problem:

Rpm's python3 API has been totally braindamaged all this time but people are only noticing now that it's starting to get used. 

We're changing rpm to return all string data as surrogate-escaped utf-8 python strings everywhere (instead of bytes with unknown encoding that the API doesn't otherwise even accept, see bug 1631292). This makes most rpm-scripts written for python2 just work with python3 too (from the rpm pov).

Most software that has kept python2 compatibility are automatically compatible with the fixed API, but unfortunately python3-only users like atomic need fixing for the new behavior.

There's at least one place in atomic that is affected, there's an explicit .decode() in _get_rpms() function.

As the broken rpm versions are widely in use, it's best to keep compatibility with both initially. Basically replace all the hardcoded .decode("utf-8") calls to header data with something like this:

def b2s(data):
    if isinstance(data, bytes):
        return data.decode('utf-8')
    return data