Make pagure-ci use python-jenkins to work with newer Jenkins
    
    Jenkins now may request the equivalent of a CSRF token, python-jenkins
    supports it while the approach we used so far does not.
    So start using python-jenkins instead of doing a POST http request that
    simply does not work on recent jenkins instances.
    
    Fixes https://pagure.io/pagure/issue/2380
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>