Add 'skip backport' label to skip the backport source check.

[terryjreedy]

For #211, we added another ignorable check to Bedevere - for the master GH-NNNN number for non-master PRs. Following the precedent of 'skip issue' and 'skip news' for other ignorable checks, add 'skip backport' (or 'skip GH-NNNN'?), with the same black blob, to skip the backport source # check.

[Mariatta]

GitMate.io thinks possibly related issues are #72 (Add a status check for reverted commits), https://github.com/python/core-workflow/issues/268 ([cherry-picker] [idea 💡] Extend backport sources), #267 (Automatically apply "needs backport to " labels.), python/devguide#126 (Document backport and cherry-pick labels), and #211 (Bot to check if PR is not against the master branch, and it is not a backport PR).

[brettcannon]

@terryjreedy that's not true anymore as of this commit. Now it only checks that you specified the branch name.