ci: Upgrade windows image to windows-2019

The image we currently use, vs2017-win2016, is deprecated. It's
currently subject to "brownouts" and will be removed in the future.
Switch to using windows-2019 as a replacement, as that version hasn't
yet been deprecated.

See
https://devblogs.microsoft.com/devops/hosted-pipelines-image-deprecation/#windows
for more information about the deprecation.

Author: Patater

azure-pipelines.yml

@@ -58,15 +58,15 @@ stages:
               sendCoverage: "false"
             windows_python_3_6:
               python.version: "3.6"
-              imageName: vs2017-win2016
+              imageName: windows-2019
               sendCoverage: "false"
             windows_python_3_7:
               python.version: "3.7"
-              imageName: vs2017-win2016
+              imageName: windows-2019
               sendCoverage: "false"
             windows_python_3_8:
               python.version: "3.8"
-              imageName: vs2017-win2016
+              imageName: windows-2019
               sendCoverage: "false"
 
         pool: