x509: Port to Python 3
    
    In python 3 , `bytes` has the buffer interface, and `buffer` was removed.
    
    Also, invalid padding in base64-encoded data raises a ValueError rather
    than TypeError.
    
    In tests, use pytest.assert_raises for more correct exception assertions.
    Also, get rid of unused imports in the tests
    
    Reviewed-By: Tomas Babej <tbabej@redhat.com>