gh-419: Allow unstable builders to trigger on PRs

[itamaro]

This intention of this PR is to make it possible to manually trigger also unstable buildbots on PRs using the !buildbot command (and labels).

The lists of "PR builders" are used in several places where I kept it to only stable PR builders, because those other places didn't seem related to the manual triggering of buildbots on PRs.

[colesbury]

I would really like to see this change land, not only for the nogil buildbots but also so I can test changes on AIX. The AIX buildbots are also marked as "unstable".

[itamaro]

@vstinner I am not sure how to test changes like this manually / locally, as it seems highly coupled with the details of the actual deployed buildbot master.

I think we do want to allow using a PR !buildbot ... comment to match unstable builders, but using the test-with-buildbots or test-with-refleak-buildbots labels should trigger only stable builders.

both PR labels and comments are handled in CustomGitHubEventHandler, but it appears that the builder_names argument is only used when doing matching against a PR comment. in case of a PR label, I don't see how exactly the builder names are plumbed through, and it isn't clear to me whether this change would impact the behavior of the label-based triggering, or only the comment-based triggering.