RFC: Backport candidates

[effigies]

I've created a label.

With the best of intentions with maintenance branches and bug-fix protocols, we still generally fork fixes off of master and fixes that could go into an older maintained series do not. The purpose of this label is to mark PRs that are targeting master (or a newer maint/* branch) as fixes that could go in earlier maintenance branches, if and when a new release is called for.

This way we can keep our usual process without forcing rebases of fixes onto maintenance branches, merging, and then merging the maintenance branch into master. When it's time to release a bug-fix release, we can check the label for any relevant PRs, perform the backports in a single PR.

Thoughts?

[mgxd]

I like it - labels are findable in the github api (https://api.github.com/repos/nipreps/fmriprep/pulls/2900) so we could eventually automate this as well.