[macOS] Default Python will be upgraded to 3.9

[AlenaSviridenko]

Breaking changes
Default Python will be upgraded from 3.8 to 3.9 on macOS images

Target date
Deployment of these changes will be started on November, 16 and will take a few days.

The motivation for the changes
Homebrew set Python 3.9 as a default Python formula, and there is no simple option to keep Python 3.8 as a default version.

Possible impact
Project dependencies that are not supporting Python 3.9 might be impacted.

• macOS 10.13
• macOS 10.14
• macOS 10.15
• macOS 11.0
• Windows 2016
• Windows 2019
• Ubuntu 16.04
• Ubuntu 18.04
• Ubuntu 20.04

Mitigation ways
Please, consider using actions/setup-python (for GitHub Actions) or UsePython (for Azure Pipelines) to switch Python versions based on your project requirements.

[MaksimZhukov]

Hello everyone!
The default Python version has been updated to 3.9 on the macOS 10.15 and macOS 11.0.
The macOS 10.13 and macOS 10.14 images are deployed once a month. Python 3.9 as a default version for these images will be delivered with the next image rollout which will take approximately 1-2 weeks.

I'm closing this issue, but feel free to contact us if you have any concerns.