Add "skip auto backport" label

[berkerpeksag]

Sometimes I need to manually backport a PR due to API changes. Currently, the only way I know to tell miss-islington not to create a backport PR is removing the "backport to X.Y" label. But I find "backport to X.Y" labels useful since we don't always keep the version field in the bpo issue up-to-date. Can we add a "skip auto backport" label (similar to what "skip news" etc. does)? I'm open to alternative names and as always I can implement it if people find the idea useful.

[Mariatta]

Sounds good. I think that the label should be "needs manual backport", or something like that. This will sound more like a call to action, that someone needs to do something now.