This script provides functionality to easily backport a merged pull request to
a release branch.

It relies of having a `github` API token stored in `~/.riotgithubtoken` by
default.

The script works by fetching information from the supplied **merged** pull
request. It then looks for the last release branch.
A temporary git `worktree` with a new branch is created based on this release
branch. All commits from the pull request are then cherry-picked into this
branch which is then pushed to `origin`.
It then creates a new pull request on `github` with a reference to the original
pull request. It optionally puts a comment under the original pull request to
the new backport pull request.

Co-authored-by: Gaëtan Harter <gaetan.harter@fu-berlin.de>