ICYMI: cherry-picker repo has moved out of core-workflow repo

It’s like spring cleaning

The source code for cherry_picker.py has been moved to its own repo at https://github.com/python/cherry-picker

Going forward, specific issues regarding cherry-picker can be filed in https://github.com/python/cherry-picker. Open issues for cherry-picker from core-workflow has been moved there as well.

See brief discussions on why we decided with the move.

Next step is to move blurb out to its own repo.