Azure Pipielines: Replace soon-unsupported macOS 10.13 with 10.14

[hugovk]

Azure DevOps Blog:

We have decided to remove the macOS-10.13 image because less than 5% of users use macOS High Sierra 10.13 and there is a later macOS version image – macOS-10.14 available. In addition, we will be adding the latest macOS version – Catalina or macOS-10.15 on February 3, 2020.

• https://devblogs.microsoft.com/devops/removing-older-images-in-azure-pipelines-hosted-pools/

More about upcoming changes:

• https://devblogs.microsoft.com/devops/azure-pipelines-hosted-pools-updates/

---

Does this need a news entry or a trivial file/label?

[pradyunsg]

The mobile UI doesn't let me add the label (unless I tap a lot of buttons, and it's late and I'm sleepy). This should be trivial (this isn't a user facing change that we'd call out in the news file).

Also, thanks for spotting this and filing this PR! This was definitely not on my radar. :)

[hugovk]

I only knew because Microsoft emailed me about another repo:

Based on our telemetry, your organization has used one of these images in the past month and will be impacted by this change. To ensure your pipeline isn’t impacted by this change, please update your pipelines using the following steps.

In that one, I changed it to macOS-latest (and ubuntu-latest and windows-latest) but I guess pip should pin for stability.