Bump pygithub from 1.54.1 to 1.57

Bumps [pygithub](https://github.com/pygithub/pygithub) from 1.54.1 to 1.57.
- [Release notes](https://github.com/pygithub/pygithub/releases)
- [Changelog](https://github.com/PyGithub/PyGithub/blob/master/doc/changes.rst)
- [Commits](PyGithub/PyGithub@v1.54.1...v1.57)

---
updated-dependencies:
- dependency-name: pygithub
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -2,7 +2,7 @@ boto3>=1.9
 https://github.com/docker/docker-py/archive/1.10.6.tar.gz#egg=docker-py
 dockerfile-parse==1.2.0
 multidict==5.2.0
-PyGithub==1.54.1
+PyGithub==1.57
 pyyaml==6.0
 taskcluster==44.2.2
 taskcluster-urls==13.0.1