add Python3 patch to rivet-cmphistos
I put added this to release-3-0-x in mercurial days but it wasn't merged onto default. Git doesn't let me cherry-pick the old commit for some reason, so I re-implemented it on master.
I put added this to release-3-0-x in mercurial days but it wasn't merged onto default. Git doesn't let me cherry-pick the old commit for some reason, so I re-implemented it on master.