Fix decoding errors on Python 2
    
    When changelogs contain non-ascii data, there could be a decoding
    problem on old Python version when the changelog is serialized. This was
    not exercised by the test suite.
    
    This partially reverts 86c951c4661d3af234f8a7b7e560ab88e12a3f2b, but
    only for Python 2.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>