Bump the minimal pygit2 version to 0.26.0
    
    The issue is simply that the current pagure developpers no longer
    have access to a 0.24.* release to replicate issue reported on this
    version. So we're bumping to 0.26.0 which is present in Fedora, EPEL
    (and thus RHEL and CentOS) in Mageia, OpenSUSE and Ubuntu 18.04+.
    
    Fixes https://pagure.io/pagure/issue/4030
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>