Bump gidgethub from 3.3.0 to 4.0.0 (#325)

Bumps [gidgethub](https://github.com/brettcannon/gidgethub) from 3.3.0 to 4.0.0.
- [Release notes](https://github.com/brettcannon/gidgethub/releases)
- [Changelog](https://github.com/brettcannon/gidgethub/blob/master/docs/changelog.rst)
- [Commits](https://github.com/brettcannon/gidgethub/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements.txt

@@ -1,6 +1,6 @@
 cherry_picker==1.3.2
 aiohttp==3.6.2
-gidgethub==3.3.0
+gidgethub==4.0.0
 cachetools==4.0.0
 redis==3.4.1
 celery==4.4.2