Fix duplication of checks on internal PRs (#1986)

Internal PRs (that come from branches of psf/black) match both the push
and pull_request events. This leads to a duplication of test, lint, etc.
checks on such PRs. Not only is it a waste of resources, it makes the
Status Checks UI more frustrating to go through.

Patch taken from:
https://github.community/t/duplicate-checks-on-push-and-pull-request-simultaneous-event/18012