Since both Paul Moore and Antoine Pitrou started to ask questions about a side comment I made about VSTS, I might as well start a discussion (Steve has also just emailed python-committers about this topic).
Basically Steve Dower and I have been working directly with the VSTS team to improve their Python support. Along the way they have announced they are adding anonymous access to build logs which was always a major blocker for using it in open source. VSTS gave us early access to the anonymous access that they are launching along with 20 concurrent builders which is more than what we currently have on Travis and AppVeyor.
At this point Steve has added the appropriate YAML files in the .vsts directory that control the build jobs so we can test out the integration and see what the performance is like. But based on what we have been seeing there will be no queue in getting PRs tested and that applies to Windows, macOS, and Linux (IOW better coverage and no more issues about performance or having to wait too long for CI to go green ;) . There is also other benefits like making at least Windows builds easier to release as signing those builds can now be automated.
My expectation is that once we are convinced that the VSTS builds are passing consistently and everything is working as everyone wants for e.g. a week, we can plan to turn off Travis and AppVeyor for master and 3.x branches (no one wants to bother with trying to get 2.7 to work on this and we can simply continue to rely on Travis and AppVeyor as necessary for the next 18 months). I don't think we want to run multiple CIs indefinitely on the same branch as it does take time and effort to keep CI green on a provider since they all have their own quirks.