Bot shouldn't say "we don't accept contributions" when someone closes a PR

[nedbat]

See what happened here: edx/dbt-schema-builder#54

A PR was closed. The author no longer works for edX, so was considered external. The bot added a comment (twice?) saying that contributions aren't accepted. This is just extra noise, the contribution was being cancelled already.

[nedbat]

BTW, it comments twice because the "Close with comment" button causes two events to fire (comment created and pull request closed). They are both sent to the webhook at the same time, so two requests are handled, and each sees the pull request with no comment on it, so both add a comment.