Use lower or equal release of pygit2 vs libgit2
    
    This fixes the current situation where libgit2 released a new version: 0.26.5
    which doesn't need any changes in pygit2, so there won't be a release made
    there. So we want to use the pygit2 version just below 0.26.4 rather than
    bumping to 0.27.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>