Fix couple of bytes vs strings issues in Python tests
    
    For the purposes of rpmio testing and importing public key, we're
    dealing with bytes rather than encoded strings. In the carefree days
    of Python 2 such details didn't matter, in Python 3 they cause failures.
    The signed package test still fails after this one but it's due to
    a more general issue.
    
    (cherry picked from commit 86f7898dd6a7fa8718c02675f5a7ee04ff987422)