Add Azure pipelines

I have heard very good things from @gaborbernat about Azure pipelines, and they support Linux, Mac and Windows all in one place. Given that Appveyor is often the build’s bottleneck and Travis seems to be having trouble, it’s probably a good idea to get this set up.

@vtbassmatt set up Azure pipelines for dateutil in this PR, so maybe we can use that as a template for setting it up here.

[pganssle]

@jaraco @vtbassmatt I think it would still be fine to do this in baby-steps. Start with adding tests, then we can add the coverage (either because codecov starts supporting AP out of the box or we use the token method), and then we can start cutting down the Travis and Appveyor build matrices.

Releasing with AP can come later I think. I think there’s a way to do it with secure environment variables and the right permissions, but I am not confident enough to do that myself.

[jaraco]

I see I completed the azure pipelines in fcc9680fd931645d0e6928a358d726daa1ab220e, including support for cutting releases, although I suspect now there’s a race between Travis and Azure to cut releases, so one of them will fail. I’m okay with that for now.