CI: Enable pep8 check
    
    CI can now check pep8 issues for all .py files but the ones under
    src/config (for those, there's an issue already filed[0]).
    
    [0]: https://pagure.io/SSSD/sssd/issue/3514
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3605
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>