Initial commit of Python cherry-picker bot

[Mariatta]

Closes python/core-workflow#8

[Mariatta]

An example of how this works:
Mariatta/cpython#36

1. PR that has "needs backport to X.Y" is merged
2. bot comments saying it will work on backporting it to X.Y
3. backport PR created
4. If there are errors, bot comments back saying it couldn't do the backport.

The bot account is: https://github.com/miss-islington

The bot does not have direct write access to Python/cpython. It will do the backport on its own forked repo.