Fix flake8 complaints
    
    E721 do not compare types, for exact checks use `is` / `is not`, for
    instance checks use `isinstance()`
    Conditions in the method `_list_branches` were switched because:
    `issubclass(git.RemoteReference, git.Head)`
    
    Signed-off-by: Ondrej Nosek <onosek@redhat.com>