Bug: 500 error when bedevere received "commit.pushed" webhook events.

[Mariatta]

I'm seeing several errors in the webhook deliveries.

Sep 08 09:56:14 bedevere heroku/router at=info method=POST path="/" host=bedevere.herokuapp.com request_id=e201214d-1c1d-44fa-8ec6-38a3d34a1d09 fwd="140.82.115.81" dyno=web.1 connect=0ms service=1349ms status=500 bytes=188 protocol=https
Sep 08 09:56:14 bedevere app/web.1 Traceback (most recent call last):
Sep 08 09:56:14 bedevere app/web.1 File "/app/bedevere/main.py", line 40, in main
Sep 08 09:56:14 bedevere app/web.1 await router.dispatch(event, gh, session=session)
Sep 08 09:56:14 bedevere app/web.1 File "/app/.heroku/python/lib/python3.10/site-packages/gidgethub/routing.py", line 89, in dispatch
Sep 08 09:56:14 bedevere app/web.1 await callback(event, *args, **kwargs)
Sep 08 09:56:14 bedevere app/web.1 File "/app/bedevere/stage.py", line 154, in new_commit_pushed
Sep 08 09:56:14 bedevere app/web.1 await request_core_review(
Sep 08 09:56:14 bedevere app/web.1 File "/app/bedevere/stage.py", line 272, in request_core_review
Sep 08 09:56:14 bedevere app/web.1 await stage(gh, issue, blocker)
Sep 08 09:56:14 bedevere app/web.1 File "/app/bedevere/stage.py", line 99, in stage
Sep 08 09:56:14 bedevere app/web.1 await _remove_stage_labels(gh, issue)
Sep 08 09:56:14 bedevere app/web.1 File "/app/bedevere/stage.py", line 91, in _remove_stage_labels
Sep 08 09:56:14 bedevere app/web.1 await gh.delete(issue["labels_url"], {"name": stale_name})
Sep 08 09:56:14 bedevere app/web.1 File "/app/.heroku/python/lib/python3.10/site-packages/gidgethub/abc.py", line 295, in delete
Sep 08 09:56:14 bedevere app/web.1 await self._make_request(
Sep 08 09:56:14 bedevere app/web.1 File "/app/.heroku/python/lib/python3.10/site-packages/gidgethub/abc.py", line 121, in _make_request
Sep 08 09:56:14 bedevere app/web.1 data, self.rate_limit, more = sansio.decipher_response(*response)
Sep 08 09:56:14 bedevere app/web.1 File "/app/.heroku/python/lib/python3.10/site-packages/gidgethub/sansio.py", line 378, in decipher_response
Sep 08 09:56:14 bedevere app/web.1 raise exc_type(*args)
Sep 08 09:56:14 bedevere app/web.1 gidgethub.BadRequest: Label does not exist

[Mariatta]

This could be related to the issue where awaiting core review label were added after merging. Example python/cpython#109082

I'm looking into this.