Bump cachetools from 3.1.0 to 3.1.1 (#232)

Bumps [cachetools](https://github.com/tkem/cachetools) from 3.1.0 to 3.1.1.
- [Release notes](https://github.com/tkem/cachetools/releases)
- [Changelog](https://github.com/tkem/cachetools/blob/master/CHANGES.rst)
- [Commits](tkem/cachetools@v3.1.0...v3.1.1)

Author: dependabot-preview[bot]

requirements.txt

@@ -1,6 +1,6 @@
 cherry_picker==1.2.2
 aiohttp==3.5.4
 gidgethub==3.1.0
-cachetools==3.1.0
+cachetools==3.1.1
 redis==3.2.1
 celery==4.3.0