[oslo] Please do not merge py27 removal patches yet

This is mostly directed at the Oslo cores, some of whom have already been -1'ing these patches (thanks!). There have been some discussions in the tc channel, but I'm not sure we've actually talked about it within the team yet. The main thing is that Oslo needs to wait until all of our consumers have stopped testing py27 before we do. Otherwise we may accidentally merge a py27-incompatible change and break other projects' gate jobs. Some patches to drop py27 jobs have already been proposed, and in lieu of someone procedural -2'ing all of them I'm hoping we can just have an understanding that these can't merge yet. I'll send a followup when we're ready to proceed with the py27 removal work. Thanks. -Ben