warning in the MacOSX CI

Running the MacOSX py38 CI job gives us this warning:
```
##[warning]This pipeline uses a Microsoft-hosted agent image that will be removed on March 23, 2020 (MacOS-10.13). You must make changes to your pipeline before that date, or else your pipeline will fail. Learn more (https://aka.ms/removing-older-images-hosted-pools).
```
